The documentary goes into detail about what happened on May 31, 2014, when the two girls, then 12, allegedly lured Payton into the woods near Waukesha, Wisconsin and stabbed her over and over again.
Credit: HBO
The parents of a 15-year-old girl accused of stabbing a friend 19 times in the name of Slenderman - a fictional, blank-faced figure - have spoken about the attack.
Bill and Kristi Weier, whose daughter Anissa will be tried with her friend Morgan Geyser for attempted murder this year, revealed their side of the story for the first time.
The couple spoke with ABC News on Monday, saying their daughter expresses "remorse" and often "vents" during visits.
They also revealed details about the day of the attack, most notably what stabbing victim Payton Leutner said.
"Payton stated that she thought Morgan was her friend," Kristi Weier said. "I pray every day that Payton finds true friendship."
The interview comes as filmmakers prepare to release the documentary Beware the Slenderman, based on the attack.
The documentary goes into detail about what allegedly happened on May 31, 2014, when the two girls, then 12, are accused of luring Payton into the woods near Waukesha, Wisconsin and stabbing her over and over again.
The film includes audio from a 911 call and police footage from inside the interview room as Anissa reveals why she did what she did.
"I was really scared knowing that Slenderman could easily kill my whole family in three seconds," she tells an officer.
Despite their age, the two girls are being tried as adults. During Monday's interview, Bill Weier said that decision should be reversed.
"The laws in the state of Wisconsin are outdated," he said. "If you look at the way the laws are now, the law has not advanced with what we know about juvenile brain development."
Weier said he and his wife struggle every day coming to terms with what happened.
"It was really surreal from the time I got the phone call to the time I realised there were more things going on than we were originally led to believe," he said.
"You struggle with processing what you're finding out about what your child is being suspected of. If [the victims' parents] were here face-to-face I would tell them I'm sorry.
"I'd tell them I'm thankful that Payton survived and that for as much as they are struggling, we are struggling equally with trying to process not only what happened to their daughter but to our daughter."
The two accused pleaded not guilty by reason of mental illness. If found guilty, they could face up to 65 years in prison.
Payton continues to recover from the wounds, both physical and mental. She returned to school one year after the attack and is carrying on her life as normal.
Her parents were approached by HBO to be a part of the documentary. Family spokesman Stephen Lyons told news.com.au he met with a producer ahead of the filming and declined to be involved.
