Wayne B. Williams, serving a life sentence for slaying two of 28 young ghetto blacks found murdered here, was a homosexual who killed his victims to achieve a sense of power and "sexual gratification," although there never was evidence of sexual contact, Georgia's crime lab director told visiting police chiefs today.

"You can't separate power from sex," said Larry Howard, a forensic expert whose lab fiber analysis proved crucial in tying Williams to his victims. "Sexual gratification includes a sense of power," he said, "so in a broad sense, yes, there was sexual gratification" in the killings.

It was a rare public postmortem on the what the police believe was the motive for the murders that terrorized this southern city for nearly two years, with key players in the case holding forth for fellow lawmen at the 89th annual conference of the International Association of Chiefs of Police. Authorities say Williams is linked to 26 of the 28 killings.

Williams was convicted of killing Nathaniel Cater, 27, a day laborer with a drinking problem, and Jimmy Ray Payne, 21, a convicted burglar who yearned to be a singer. But many Atlanta residents still wonder why a near-brilliant young man would murder poor ghetto youths of his own race. He has consistently said he is innocent and is appealing his convictions.

Prosecutors portrayed Williams, 23, as a self-styled music promoter who sought to absolve his own sense of failure by committing black genocide. But Howard said he believed it boiled down to power. "He was a nobody who wanted to be a somebody," said the scientist. "But in order for him to feel he had power, he had . . . to show others he had power."

The killer played cat-and-mouse with police, leaving bodies in the open to be discovered, later dumping them in rivers after newspapers revealed that physical evidence had been found on the victims. He got bolder, killing older victims. It was a case that hung by hundreds of threads taken from the bodies.

"We didn't have any bullets or fingerprints, only what we got off the bodies," said Larry Peterson, a crime lab microanalyst.

Because so many fibers matched, the scientists advised investigators to look for a single killer, he said. A crime lab profile revealed for the first time today predicted the killer would be a "young, black male on the fringe of his peer group," said Howard.

"We knew he was ego-deficient because he responded to the newspapers," he said. "He put bodies in the open so they could be found. He enjoyed our puzzlement. We knew he was a loner because no one responded to the $100,000 reward."

Howard implied that police deduced the killer was black because "we knew he killed from a positon of trust because autopsies showed the victims were killed [most by aspyxiation] with very little struggle. And as the suspect got braver, he killed individuals who were stronger. We could see him gain confidence."

After Williams' arrest, the latest forensic technology was used by Atlanta authorities and by the FBI in Washington to match fibers from the victims with others from Williams' carpet, bedspread, dog and cars.

Williams denied any prior contact with the victims, but the tiny fibers proved he lied, said Howard.

"The fibers made the association," he said.

Lights were dimmed as Georgia's two high-tech heroes used a mannequin to show 100 lawmen how to recover such fibers from bodies without contaminating evidence. "Start at the head with a pair of tweezers and don't forget the soles of the feet," lectured Howard. "You've got to have a strong stomach."

Then on came the color slides of the victims, several bloated beyond recognition, and carpet fibers from the bodies and Williams' home. The matchups were remarkable.

Some of the material presented today was not shown at the trial.