There were disturbing new details Wednesday in the homicide case of Mark O'Donnell, who is accused in the killing of 14-year-old Ebony Dorsey.

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Prosecutors said they believe the Wissahickon High School student was killed at the hands of her mother's boyfriend.

Prosecutors are asking a judge to consider what they call new evidence against O'Donnell. In court documents filed this week, prosecutors laid out the new evidence and much of it is graphic.

"I don't remember doing it but I know that she is dead and it was at my hand," O'Donnell told reporters in December when he was arrested.

The first-degree murder suspect said on the day of his arrest that he killed Dorsey but denies raping her.

But NBC 10 has learned prosecutors have new evidence against O'Donnell.

"It's critical on the issue of the sexual assaults. In order to prove the underlying sexual assaults we feel we need this evidence," Montgomery County District Attorney Risa Vetri Ferman said.

Prosecutors said O'Donnell, who was dating Dorsey's mother Danielle Cattie, brought Dorsey to his Plymouth Township apartment on the evening of Dec. 6.

Prosecutors said he left Dorsey there to baby-sit his child and returned to Cattie's Whitpain Township home.

Prosecutors said on the morning of Dec. 7 O'Donnell returned to his apartment where he raped, beat and strangled Dorsey and then left her body stuffed in a plastic container in a relative's back yard.

"He's going to acknowledge that he was involved in her death. But he adamantly denies any kind of sexual contact," Tom Egan, defense attorney, said.

Prosecutors have submitted evidence suggesting O'Donnell participated in unconventional sexual acts.

They recovered a number of DVDs and sex toys from the Cattie home and are asking a judge to allow those DVDs, toys and a number of pornographic images from a computer found in the O'Donnell home to be allowed into evidence.

Court documents suggest the computer in the O'Donnell home was used hours before the killing and prosecutors allege O'Donnell had images on that computer that showed a man and woman engaging in a sex act and the man was pulling a robe around a woman's neck.

Prosecutors said when Dorsey's body was discovered her pajama bottoms were wrapped around her neck -- in the same way as the computer image.

"There is a similarity in the actions both in looking at photographs and the things in which he was interested and in the way that Ebony was killed," Ferman said.

O'Donnell's lawyer said he plans to fight the new allegations.

"I need to know who it is that signed in (to the computer), what log-in names and passwords were used and so forth because I don't know who was using that computer at that time," Egan said.

A judge is set to hold a hearing on the issue within the next couple of days and will determine if the evidence will be allowed at trial.

O'Donnell is charged with a number of crimes including first-degree murder and prosecutors are seeking the death penalty.