She was a well-liked honor student, a frequent caregiver for her three younger siblings - "a little mom," in the words of one family friend.

Soon to turn 15, Ebony Nicole Dorsey seemed to be beating long odds. Cocaine was a frequent presence in her Whitpain Township home, brought there, court records say, by her mother and her mother's boyfriend, Mark P. O'Donnell.

Early Friday, the odds overtook the outgoing Wissahickon High School freshman, according to prosecutors.

In what authorities allege was a savage, drug-fueled assault, O'Donnell beat, sexually assaulted and strangled the teen as she babysat his own 4-year-old daughter. He then stuffed her body into a blue, plastic tub and concealed it outside the home of a relative, covering it with sticks and fallen leaves.

The assault was at least partly witnessed by the little girl, who called out "Daddy" as O'Donnell finished strangling Dorsey with her own pajamas, he told police.

The teen had been driven by O'Donnell to Plymouth Township on Thursday night to stay overnight at his apartment and watch his little girl. O'Donnell, meantime, spent the night in Dorsey's home smoking crack cocaine with the babysitter's mother - described in court papers as his girlfriend of four years.

Before dawn Friday, prosecutors say, O'Donnell returned home to find Dorsey changing his 4-year-old's pull-up diaper. He flew into a rage and killed her, they say.

"By everyone's account, this was just a terrific young lady," said Risa Vetri Ferman, Montgomery County first assistant district attorney. "The death that she suffered was a very violent, vicious death."

O'Donnell, 48, of the 1900 block of Sandy Hill Road, was charged yesterday with murder, involuntary deviate sexual intercourse, indecent assault, tampering with evidence, and abuse of a corpse. He also was charged with felony drug possession. Despite his relationship with Dorsey's mother, O'Donnell is married. His wife was out of town.

O'Donnell was being held last night without bail in the Montgomery County prison. On his way out of his arraignment yesterday, he said he caugh the teen molesting his daughter, not changing her diaper.

O'Donnell said to a crowd of reporters that he killed the teen. He added: "I don't remember doing it, but I know she was dead at my hands."

Dorsey had not been reported missing by her mother, Danielle Cattie, until Friday evening, despite the teen's absence from school that day. After a search by more than 75 police officers and firefighters, her body was found Sunday afternoon outside the home of O'Donnell's nephew in the 1300 block of Union Meeting Road in Blue Bell, Whitpain Township.

Police said the body had been crammed into a plastic storage tub measuring only 221/2 by 16 by 151/2 inches.

O'Donnell, who had been interviewed by police on Saturday, was arrested late Sunday, hiding at the home of other relatives in Lower Gwynedd Township.

No charges were filed against Cattie yesterday. In a probable-cause affidavit, police describe her as "a self-admitted user of cocaine" who had known O'Donnell, her supplier, for seven years, and had dated him for at least four.

Montgomery County Court records show that Cattie, now 34, pleaded guilty in 1998 to felony possession of crack cocaine, and was placed on five years' probation. At the time of Cattie's arrest, Ebony Dorsey was 4, and her mother was pregnant with the third of her four children.

Despite that at-risk home life, Dorsey was remembered by her peers at Wissahickon as an accomplished, friendly student.

"She's very funny and outgoing," said classmate Bekkah Fernandez, 14, of Blue Bell. "A lot of people have been crying and just doing what they can to comfort people."

Extra school counselors were called in, a moment of silence was held, and white paper banners spanned the lobby walls outside the auditorium, where classmates scrawled memories, condolences and messages to "Babygirl" and "EB-BONE."

Principal William Hayes said he had visited Dorsey's mother yesterday to assure her that "we were thinking about her."

O'Donnell was described by prosecutors as a private-duty nurse in West Philadelphia. Cattie described him to police as a frequent drug abuser who turned violent when high - a man who had choked her, pulled her hair and thrown her to the ground.

He had smoked "a significant amount of crack cocaine" at her house, according to a probable-cause affidavit, and still was "wired" when he left at 5:30 a.m. Friday to pick up Dorsey and his 4-year-old, Kyra.

By 6 a.m., a neighbor reported hearing "a lot of noise" in O'Donnell's apartment upstairs.

After the girl disappeared, the affidavit says, O'Donnell gave police a series of conflicting statements over two days before confessing after the body was found.

O'Donnell told investigators he had arrived home at 5:45 a.m. to see Dorsey changing Kyra on the sofa. He yelled at her for changing the toddler "during the night," the affidavit said, and began beating her in the face.

He then used her pajama bottoms to strangle her in Kyra's bedroom, the affidavit says, stopping only when he heard his daughter say, "Daddy." O'Donnell then "realized . . . that Ebony was no longer breathing," the affidavit said.

An autopsy found that Dorsey had died of blunt-force trauma and asphyxiation. It also found anal injuries "consistent with penetration by a foreign object and sexual assault," the affidavit said.

At Wissahickon High School, students seem determined to remember the freshman in a less horrific light.

Some seniors want to use funds raised from their senior projects - a graduation requirement - to establish a scholarship. Students and advisers are mulling whether to donate proceeds from this Saturday's semiformal dance to her family.

Until then, music will suffice.

Tonight's winter choral concert will be dedicated to Ebony Dorsey.

Contact staff writer Kathleen Brady Shea at 610-701-7625 or kbrady@phillynews.com

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