The parents of a teenage girl accused of attempting to murder her friend in a wood in Wisconsin, US have spoken for the first time since the brutal attack – saying their daughter wanted to prove a fictional character called "Slender Man" was real.

Payton Leutner, now 14, was aged just 12 when she was stabbed 19 times and left for dead during a vicious attack near the city of Waukesha on 31 May 2014.

Suffering stab wounds to her arms, legs and torso, she survived after crawling from the woods to a nearby cycle path where she was found by a member of the public.

Her two friends, Morgan Geyser and Anissa Weier, are accused of carrying out the attack and are due to stand trial for attempted murder.

In police interviews, Weier and Geyser, then aged 12, said the attack on Leutner was a sacrifice to "Slender Man" (also known as Slenderman) – a ghost-like fictional bogeyman that went viral online after being created for a competition.

Geyser had told officers she was encouraged to attack Leutner after "Anissa said that we had to ... she said that [Slender Man would] kill our families".

Weier had told officers Slender Man "can be anywhere from six feet to 14 feet tall," adding: "I was really scared knowing that Slender could easily kill my whole family in three seconds."

In their first interview since the attack, Weier's parents said they were still struggling to come to terms with the attack.

"They thoroughly believed that Slender Man was real and they wanted to prove that he was real," mother Kristi told ABC's Good Morning America.

"We've never seen her watch videos or read stories or say 'hey, look what I found on the internet'. She was just typical.

"Compared to the other three children (her siblings) she didn't show any signs of... disbeliefs or anything."

The parents have not spoken with the victim's family.

But father Bill said: "If they were here face to face I would tell them I'm sorry, I would tell them I'm thankful Payton survived.

"And I would tell them that for as much as they are struggling with trying to process this and what happened to their daughter, we are struggling equally trying to process this with not only what happened to their daughter but to our daughter."

The two accused girls remain in custody and have pleaded not guilty to attempted murder on grounds of mental illness. They each face up to 65 years in prison if convicted in an adult court.

Bill and Kristi Weier described their daughter as remorseful and have been fighting to prevent her being tried as an adult, saying "outdated" laws on criminal age of responsibility have not kept pace with new research on juvenile brain development.

Children as young as 10 can be tried as adults in Wisconsin.

Weier's parents are to feature in an upcoming HBO documentary, Beware the Slenderman, exploring how the internet phenomenon may have led to the near-death of Payton.

Slender Man, a fictional supernatural character, was created by Eric Knudsen in 2009 as part of an online forum competition to edit everyday photos to appear paranormal.

He is often depicted as a thin, unnaturally tall man with a featureless face, wearing a black suit.

After going viral online, the character took on a life of its own with fans writing stories around the character often featuring him stalking or abducting people.

HBO's Beware the Slenderman will be broadcast on Monday (23 January).