After conducting 2,000 interviews and devoting hundreds of hours to their investigation, Prince William County police say they have a chief suspect in the slaying of 15-year-old Lisa Renee Triggs last year but lack the evidence to justify bringing charges.

Triggs, a sophomore at Osbourn Park High School near Manassas, had been shopping at Center Plaza and persuaded her mother to allow her to walk home alone to meet friends there. She was last seen about 7:30 p.m. on May 21, 1986.

She was found three days later face down in a creek behind the shopping center, dead of blows to the head and neck inflicted with a blunt object.

As many as a dozen people came under suspicion in the months after the slaying, but police said last week that the investigation is focusing on one man, described as in his twenties. He remains free for lack of evidence -- in particular, a witness, according to J.G. Kuehn, the chief investigator in the case.

"Somebody knows more than they are telling," Kuehn said. "As frequently as that path {the area where the body was found} is used, somebody had to see or hear something. Whether they are afraid to come forth, we just don't know." A $2,500 reward is being offered for information in the case.

An FBI profile of the sort of person who might have committed the slaying suggests a possible history of sex offenses, Kuehn said. But he declined to say whether the girl was sexually assaulted.

Deborah Triggs, 38, Lisa Triggs' mother, says she writes poetry as a means of coming to terms with the girl's death. One of the poems, printed in a local newspaper on the anniversary of her daughter's death, read in part: "You murdered her and you too must die. The all-seeing eye knows you live a lie."