A Manassas man charged with murder in the strangulation of a 15-year-old Chantilly girl told police he killed the girl unintentionally as they were having sex behind a shopping center, a Fairfax County police investigator testified yesterday.

During a preliminary hearing, Investigator Edward R. Guckenberger said that Preston C. Brown, 21, an unemployed landscaper charged in the death of Rebecca Lynette Reese, made the statement after five hours of questioning.

Brown was arrested Aug. 6, one day after Reese's body was found behind town houses in the 4000 block of Mason Dixon Drive in Chantilly in western Fairfax County. Reese's body, clad only in a black T-shirt and a collar, was propped against a chain-link fence, Guckenberger testified.

Jonquil Reese, the girl's mother, had reported her daughter missing that morning. Jonquil Reese said her daughter left their house in Chantilly about 8:45 p.m. Saturday, saying she was going to meet some friends at a local grocery. "She said she'd be back in 20 minutes; she was just going down to the store," her mother said.

Guckenberger said Brown was arrested on a warrant for failing to appear in court on charges of sexual battery and public drunkenness.

Guckenberger testified yesterday that Brown gave this account during the first hours of questioning: He saw Rebecca Reese about 9:30 Saturday night. She was going to the grocery. He was going to visit friends. The two stopped at a playground and talked to some friends. He invited her to go to "the rocks," a place behind Chantilly Shopping Center where teenagers hang out. On the way there, she changed her mind, saying her mother "has been on me about staying out late." She gave him a cigarette and he went home.

Guckenberger challenged that account. "Finally, he gets to the point where he says, 'I'm going to be truthful,' " Guckenberger testified. At 10:22 p.m., the investigator said, he turned on a tape recorder.

Then, Guckenberger testified, Brown recounted that he and Rebecca Reese were having sexual intercourse behind the shopping center. At some point she wanted to stop. "He says no . . . . He takes his right hand and puts it around her throat. He admits she was gagging and choking. When he was done, she was not breathing."

Brown checked Reese's pulse, Guckenberger testified. "He panicked. He started carrying her, then, finally, he put her back where she was found."

William Reichhardt, Brown's court-appointed attorney, did not present evidence. "This is obviously a tragedy," he said after the hearing. "We're at the preliminary stages."

Juvenile and Domestic Relations District Court Judge Gaylord L. Finch Jr. sent the case to the grand jury, which will meet Monday.

Brown sat silent during the hearing. He is being held on $155,000 bond.