A 21-year-old Manassas man was charged with murder yesterday after an autopsy showed that a 15-year-old Chantilly girl had been strangled, police said.

The body of Rebecca Lynette Reese, wearing only a black T-shirt and a collar, was found Sunday morning behind some town houses by a man walking a dog. At the time police described her death as suspicious because the cause of death was "not readily apparent."

Monday night, Preston C. Brown, an acquaintance of Reese's, was arrested on an unrelated outstanding warrant, police said. He was charged with murder after the cause of death was determined by an autopsy, they said.

"I was kind of hoping she didn't suffer, but she did," Todd Reese said yesterday after learning how his sister died. "Now all I can do is grieve for our loss."

Police spokesman Michael Proffitt said Brown's name "surfaced early in the investigation" as someone who might have been with Rebecca Reese Saturday night after she was reported missing by her mother. "They were part of the same group of kids that hung out together," he said.

Brown was arrested by police Monday night on Oyster Point Court in Reese's neighborhood on a charge of failing to appear in court on a sexual battery charge, Proffitt said. Brown was being held at the county jail yesterday on $150,000 bond for the murder charge, he said.

Police said a motive for the slaying was "not being ascribed at this time."

Todd Reese, who was crying yesterday at the front door of the family's two-story house in the 13000 block of Adelphi Court, said he had never heard of the man charged with killing his sister. Reese said his mother, Jonquil Reese, was staying with relatives for a few days. Family acquaintances said Reese's parents were divorced several years ago. Reese would have been a junior this fall at Chantilly High School.

The girl's mother, who talked to reporters briefly Sunday, said her curly-haired daughter had left home about 8:45 p.m. to meet a friend at a local grocery store.

"She said she'd be back in 20 minutes; she was just going down to the store," the mother said. Fearful her daughter had run away from home, as she had done a few times before, she called police shortly after midnight.

Brown, who lives in the 7800 block of Community Court in Manassas, is unemployed, police said. According to Fairfax court records, his most recent job was listed as "landscaping."

Brown was arrested April 24 on charges of sexual battery and drunkeness in public, and a warrant was issued when he did not appear for a court hearing on the charges several days later, according to court records.

After violating the terms of his probation on a 1987 burglary charge, for which he was ordered to serve one year of a three-year sentence, Brown was ordered by a Fairfax Circuit Court judge to serve the rest of his sentence.

Reese's death is the eighth homicide reported in the county this year, according to police. There were 28 slayings last year, they said.