US hostage Aid worker Abdul Rahman 'killed by IS'

A video posted online claims to show that Islamic State militants have killed the captured US aid worker Abdul-Rahman Kassig. The video shows a masked man standing over a severed head which he says is that of Mr Kassig. The US says it is working to determine the authenticity of the video, which also shows a mass beheading of 18 Syrian troops.

Mr Kassig, also known as Peter, was captured last year. His family, who live in the US state of Indiana, said they were awaiting confirmation of the reports about their "treasured son". "We prefer our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause," the family said in a statement. 

This is perhaps the most elaborate and graphic of all the murder videos posted online by IS. Much of it is taken up with a recent history lesson on Iraq and Syria as seen through the eyes of the jihadists. But the latter part shows the beheading of 18 Syrian prisoners in revolting, lingering detail. Unlike earlier videos, this one revels in gore. Amongst the boiler-suited captives murdered in cold blood is a man IS says is the former US soldier Peter Kassig, who converted to Islam and changed his name to Abdul-Rahman. Neither his conversion, nor the fact that he was helping refugees when he was captured a year ago, appear to have saved him.

Mr Kassig's murder is a sign of frustration that IS militants are unable to hit back at the coalition air strikes that have driven them off key sites like the Mosul and Haditha dams, and prevented them from seizing the town of Kobane.

Unlike similar videos released in the past, the latest shows the faces of many of the jihadis. It also at one point gives a specific location - near Dabiq in Aleppo province, with an identifiable village in the background.

The latest video did not show the person identified as Mr Kassig being beheaded. Also unlike previous videos, it did not show other Western captives or directly threaten to behead anyone else.

The US National Security Council said it was investigating the video, and would be "appalled by the brutal murder" if it was confirmed. IS has previously murdered four Western hostages - British men Alan Henning and David Haines, and US journalists James Foley and Steven Sotloff. Those killings were carried out by a man believed to be British. That man bears a resemblance to one of the masked militants pictured in the video purporting to show Mr Kassig.

Mother: Slender Man Stabbing Victim Feared Specter

 A 12-year-old Wisconsin girl who was stabbed 19 times by classmates trying to please a fictional horror character had feared the specter known as Slender Man before the attack, her mother said in an interview aired Friday night. According to investigators, the two eighth graders, Morgan Geyser and Anissa Weier from Waukesha, Wisconsin, had decided they wanted to be with Slender Man, a fictional horror character born from a dark corner of the Internet. In stories they read about him online, Slender Man kidnaps and kills children. Not only did the girls believe he was real, police said, they were convinced the only way to get in his good graces was to kill someone too. But Slender Man isn't real. And their friend, a 12-year-old girl named Payton Leutner, survived being stabbed with a large kitchen knife 19 times. The Slender Man stabbing case would go on to not only rock the Waukesha community, but shock people across the United States and internationally. It would also put a spotlight on Slender Man's massive cult-like following. In an ABC News "20/20" exclusive, Payton and her parents, Stacie and Joe Leutner, opened up for the first time about recovering from the horrific ordeal. It was May 31, 2014. Around the same time police say Morgan and Anissa were leading Payton into the woods, Payton's mother Stacie was in the backyard of their family home painting an old patio set. Her 9-year-old son Caden was inside playing, and her husband Joe Leutner was at work.

Suddenly, Stacie looked up and saw a police officer and another woman, a detective in street clothes carrying a badge and a gun, walking up the side of her house. "They asked me, 'Are you Payton's mom?'" Stacie said. "The first thing that goes through my mind is my husband and my daughter are both gone. Something has happened to one of them because that's the only reason a uniformed officer and a detective comes to your house on a Saturday morning. Something has happened to somebody that I loved." They proceeded to tell her Payton had been stabbed. Then, Stacie said, the detective's phone rang. When she hung up, she told Stacie that her daughter was bleeding around the heart and she needed to get to the hospital. That's when Stacie made the worst phone call of her life to her husband, Joe. "I said, 'Morgan stabbed Payton, you need to come home,'" she said. After that, Stacie said she went into the living room where saw her son playing, and broke down. "I just looked at Caden, and I said, 'Morgan stabbed Payton. We have to go to the hospital right now,'" she said. "And Caden started crying." Waukesha Memorial Hospital is a short drive from the Leutners' house and Stacie got there in a few minutes. When she walked into the trauma room, she saw Payton lying on a gurney. "She was pale as a ghost," Stacie said. "She was terrified, she was crying, she couldn't breathe." Seeing her mother, Payton reached out for her and Stacie rushed over and hugged her.

"And I said, 'you're going to be OK, it's going to be fine,'" she said. "But I could see that she was covered, her arms, her legs, abdomen, were covered in stab wounds. And all I hear is, 'there's five on her arm, there's seven on her leg,' and I'm thinking, 'seven? What?'" It was the trauma nurses, counting the stab wounds on her daughter's body. "And one of the nurse's said, 'OK, I count 19,' and the second nurse said, 'I count 19 as well,'" Stacie said. The wounds on Payton's arms and legs were to soft tissue and didn't pose an immediate threat, but two stab wounds were to major organs. In one, the knife had nearly penetrated her heart, but missed a major artery by less than a millimeter. "If the knife had gone the width of a human hair further, she wouldn't have lived," said Dr. John Keleman, who operated on Payton.

The other wound went through her diaphragm, cutting into her liver and stomach. To repair the wounds, Keleman had to crack open Payton's chest. She was in surgery for six hours. After the surgery, Payton was intubated and communicated by writing on a pad of paper. Two days after the incident, she was able to speak, but talking was still painful because of the damage to her diaphragm. Stacie said Payton's first words to her parents were "I want to go home." Then, she asked something else. "She asked, 'Did they get them?'" Stacie said. Her father, Joe, assured her that the police had arrested Morgan and Anissa, and they wouldn't be able to hurt her anymore. He tried to keep a brave face for his daughter, Joe was angry. "This is my little girl who's lying there, who just said something incredibly horrendous happened, and now she's scared for her life, because these two people could potentially find her again," he said. Stacie asked her daughter if she remembered what happened, "and she said, 'I remember everything.'" Growing up, Stacie said Payton and Morgan were close. They spent all day together at school, and then called each other as soon as they got home and talked for hours. When they weren't talking on the phone, they were messaging each other online. Joe said he thought it was a typical friendship for two pre-teen girls who were best friends, but Stacie said she worried that Morgan seemed a bit more controlling, and sometimes the two girls would get into arguments. "Payton would get so upset about them," she said. "As a mother, I don't like seeing her upset over these arguments." Stacie said she would try to reconcile disputes, telling Payton to email Morgan when she made her upset. "I would tell her, 'you need to have a conversation with her,'" she said. "They were just little things to me." But Morgan also had another best friend, Anissa Weier. Stacie said Anissa entered the picture sometime when the girls were in sixth grade, but Payton wasn't close with her. "Payton had casually mentioned Anissa's name before," she said. "She maybe ate lunch with Morgan and Payton once in a while, she says now." Anissa and Morgan shared a friendship outside of the one Morgan had with Payton, a friendship that concerned a dark character they learned about on the website Creepypasta.com called Slender Man. Slender Man is not so unlike characters from other horror tales that have been around for decades - the Boogeyman, the Man With the Hook or Bloody Mary, for instance. Slender Man is a tall, thin, faceless figure dressed in a black suit with long tentacle-like arms he uses to abduct children. Slender Man was first sighted on the Internet in 2009, when someone with the username Victor Surge posted two pictures of a tall, faceless man in the background of a group of children, implying that something bad was about to happen to them.

"What got people talking about the Victor Surge photos was this kernel, this sort of seed, that, 'ah, 'what's going on here?'" said Dr. Eric Newsom, an assistant professor at the University of Central Missouri who wrote about Slender Man in his doctoral dissertation. "We know that Slender Man abducts people but we have no idea what he does with them after that," Newsom said. Over time, fans have added their own made-up stories about Slender Man's horrific dealings, building on his character. There are now thousands of Slender Man copycats depicting a mysterious and creepy man lurking in the background, including video web series with millions of views on YouTube and a video game called "Slender: The Eight Pages." The game was released in June 2012 and its official website crashed after too many people tried to download it. According to Payton, Stacie said Morgan and Anissa talked about Slender Man constantly. Morgan would send Payton links to different Slender Man stories that she wanted Payton to read. Payton was terrified of them, and Stacie said she told her to tell Morgan she was scared and wanted her to stop sending her stories about Slender Man. Stacie even offered to look at the Creepypasta site with her. "We opened up the computer one night and sat and looked at the site together, and I said, 'Payton, let's just read one of these and you tell me, does this sound like this could possibly be real?'" Stacie said. "And she said, 'No, but Morgan--Morgan knows he's real.'" Eventually, Payton stopped talking about Slender Man. Stacie said she assumed her daughter had finally told Morgan she wasn't interested and that was the end of it. But by that point, police say Morgan and Anissa had started hatching a plan to kill Payton. According to investigators, Morgan and Anissa had discussed different ways of killing their friend. They finally decided to invite Payton to a sleepover at Morgan's house on Friday, May 30, 2014, to celebrate Morgan's 12th birthday. Police say they planned the murder for 2 a.m. while Payton slept. They would duct tape her mouth, stab her in the neck and run "so they didn't have to look into her eyes," according to court documents.

Morgan invited Payton to the sleepover a month before. When Stacie asked Payton who would be there, she said Payton told her, "just me and Morgan and Anissa." So Stacie said OK. When the day of the sleepover arrived, Stacie said Payton was so excited, even "squealing with joy," and packed her American Girl doll with her things. The three girls went to Skateland, a local indoor rollerskating place, then went back to Morgan's house for the sleepover. Stacie said Payton was supposed to call her the next morning around 10:30 or 11 a.m. to pick her up. But instead of getting a phone call from Payton, two officers showed up in Stacie's backyard. According to investigators, Morgan and Anissa kept changing their plan for how they would kill Payton that night. Morgan said they decided not do it that night because "I wanted to give Payton one more day," according to the police report. Police say Anissa told them she believed after the murder she would go live with Slender Man and never see her family again.

The next day, police say Morgan and Anissa came up with a new plan -- they wanted to Payton to a nearby playground, where there was a park bathroom, according to the criminal complaint, because "the floor had a drain for the blood to go down." Before leaving for the park Morgan grabbed a knife, which she later showed Anissa under her jacket, and Anissa told police, "I thought, dear God, this was really happening," according to court documents. But in the bathroom, police say Anissa told Morgan she "couldn't do it." Anissa pointed to the nearby woods, according to the criminal complaint, and suggested they kill the victim there. "They said they were going to play hide and seek so they could distract [the victim] and kill her," the police report states. After they started the game, Anissa pushed Payton down and sat on her, but she started screaming so Anissa got up. According to the police report, Morgan and Anissa then passed the knife between them multiple times, when it finally rested in Morgan's hand. "'I'm not going [to stab her] until you tell me to,' [Anissa] stated," according to the police report. "Go ballistic, go crazy," police say Anissa told Morgan. Morgan then allegedly tackled Payton and started stabbing her, and Anissa then took a turn stabbing her, according to police. Eventually, Anissa told Payton to lie down on the ground so she wouldn't lose blood while they left to go get help. But police say they didn't go to get help. After they had gone, Payton managed to crawl from the woods to a small path. Around that same time, Greg Steinberg came down the path on his bike and stopped when he saw someone lying in the grass. "I was just on the verge of asking her if she was OK, I mean, a 12-year-old girl, she could have been out, laying in the sun, I really wasn't quite sure, I was just going to ask her and she looked up and said, 'Can you help me? I've been stabbed multiple times,'" Steinberg said. He immediately called 911 -- "Yes, she's breathing... she said she can take shallow breaths," Steinberg is heard telling the 911 operator. "She's got blood on her clothes." Dr. Brian Huckstorf, a trauma surgeon at Waukesha Memorial Hospital, said he got a call at 10 a.m. that the paramedics were bringing in a 12-year-old girl with multiple stab wounds. The girl was awake, but having trouble breathing. Huckstorf said he remembered thinking she was remarkably calm for someone who had just been through such an ordeal.

When he examined her, Huckstorf noted her heart rate was elevated and her blood pressure was low, a sign that some of her organs could be compromised. He also noted that the girl had been stabbed in what doctors call the "cardiac box," which includes the aorta, major arteries and the lungs. Patients with injuries in that area usually have a 25 to 50 percent chance of dying, he said. As Payton was fighting for her life on the operating table, police were out searching for Morgan and Anissa. In a matter of hours after the stabbing, they found the girls near an on-ramp on I-94. According to the police report, they told police they were on their way to find Slender Man so they could live with him in his mansion. Both were arrested and charged with attempted first-degree intentional homicide. The Waukesha District Attorney decided to charge the two as adults. After surgery, Payton spent the next week recovering in the hospital. When her mother asked her how she was able to crawl out of the woods, "she said, 'Because I wanted to live,'" Stacie said. At home, Payton ran fevers and still struggled to breathe for weeks. Even walking short distances would leave her winded. But after six weeks, she was able to start swimming again.

The family started getting thousands of letters and messages of support from all over the world, many people sending purple hearts - purple is Payton's favorite color. When the new school year started in September, Payton had physically recovered, and was eager to get back into a normal routine. Meanwhile, Morgan and Anissa were taken to a juvenile detention center and spent the summer in and out of court hearings. On Aug. 1, a judge declared Morgan incompetent to stand trial after two doctors testified on her mental state, saying she believes she talked to imaginary characters, including Voldemort, unicorns and Slender Man. Her next court date is Nov. 12. On Sept. 15, Anissa's attorney filed a motion arguing that she was also mentally incompetent to stand trial. A court-ordered psychologist is evaluating her before an Oct. 22 hearing. Whether the girls will ever stand trial for attempted homicide is in question. If they do, it's possible Payton will have to testify, although no one believes she is ready for that.

"We're just trying to get her to heal and it would be a long process, I think, of getting her into the stages where she's ready to go to a trial and start reliving these facts," said her attorney Michael Lauer. Life for the Leutners in some ways has returned to normal. Kids go to school, Payton does her homework. There are unpaid medical bills, and a "Heart for Healing" fundraiser page was set up for people to donate money to their family, according to their spokesperson Stephen Lyons of the Whyte Hirschboeck Dudek S.C. law firm. The firm is providing their services at no expense to the Leutners. Their help, Lyons said, has been generously covered by an anonymous third party. The Leutners say a reason they shared their story with "20/20" was so they could thank the thousands of well-wishers who have reached out and in some way supported their family through this difficult time. But for the Leutners, the biggest costs to their family aren't financial, but the weight of the horror that has changed their lives forever. "I think the hardest part for me is that Payton put this trust in this girl," Stacie said. "And Morgan knew that she was going to do this to Payton that night. She knew, and to betray somebody like that and to hurt somebody like that... Both of my kids have been robbed of their innocence. No child should ever have to go through something like this. Caden should have to watch his sister go through it, and Payton shouldn't have to go through it. It's just wrong."

Stacie said she thinks about Morgan and Anissa's parents "all the time," and wonders how they are dealing with the aftermath. Morgan and Anissa's families and their attorneys declined to comment to ABC News. "I can't imagine what they're going through," Stacie said. "I can't imagine getting that phone call, hearing these things... know it was the person that you loved and that you trusted." The Leutners hope that out of this horrific ordeal, their daughter might serve as an example for others to be better. "[Payton is] meant to do something special," Payton's father Joe said. "She's here for a reason... to show the world to treat other people better, for parents who maybe see-dig a little deeper about what their children are doing, maybe she herself is supposed to be an inspiration."