Camille Bell is still hunting for a killer. It has been more than two years since her 9-year-old son, Yusuf, was found strangled in an abandoned school a few blocks from their home in a public housing project. He was one of the 28 young blacks murdered here.

Yusuf was poor but scrappy, running errands for spare change like other street-wise youths the accused killer is said to have despised.

But Yusuf is not among the victims whose dreary lives prosecutors have paraded before a jury in the gray granite courthouse where Wayne B. Williams, 23, is on trial for two murders. Nor is Jeffrey Mathis, 11, whose skeletal remains were found almost one year after he was seen climbing into a blue car in March, 1980.

There are similarities between their murders and 10 others prosecutors have been allowed to introduce to show a pattern, but little evidence.

Still, both mothers make pilgrimages to the packed courtroom, sitting with other victims' parents among the spectators.

Outside the courtroom, Willie Mae Mathis, Jeffrey's mother, pronounces Williams guilty to any who will listen. Her daughter saw him drive past their house in a blue car the day her son disappeared, she insists.

Nearby, Camille Bell proclaims his innocence, dashing between courtroom and ghetto to track down leads for the defense team. "We've got one suspect who knew more than 14 kids and another who knew 20," she says.

Alonzo Cater, 59, a soft-spoken man with salt-and-pepper hair, keeps his verdict secret from the back row. Last week he was shocked to hear a witness testify that his son, Nathaniel, 27, a day laborer with a drinking problem, was seen holding hands with Williams outside a movie theater six hours before prosecutors say the defendant strangled him and tossed him in the river.

"I'd hate to say he did it," says Cater, "then have the jury find him not guilty. Wouldn't want that on my conscience. But I sure would like to know why he was killed."

It has taken Fulton County District Attorney Lewis Slaton six weeks and 115 witnesses to come up with a motive for mass murder. Last week he offered it, portraying Williams as out to commit black genocide.

Witnesses sketched a psycopath with a split personality, a television cameraman-turned-failed talent scout who was so filled with unrequited ambition and hate for the black street kids he recruited for stardom that he murdered them to purify his race.

Now comes Alvin Binder's turn. He is a drawling Mississippi lawyer with an attack-dog style who hinted at how he would whittle away at that portrait when the defense cranks into high gear next week. He is expected to try and scramble the pieces of Slaton's neat "jigsaw puzzle."

Binder's offensive started Friday with an ex-police recruit chipping away at the credibility of two police rookies who were on a bridge stakeout May 22 when Williams was stopped for questioning.

The two rookies were not only bored, but slept and drank on duty down by the Chattahoochee River, he said.

They were the ones who testified earlier that a loud splash alerted them to Williams' car driving slowly across the bridge. Two days later, Cater was fished from the river a mile downstream, near the spot where Jimmy Ray Payne, 21, was pulled out a month earlier.

No one saw a body go over the rail, but Williams was charged with both murders after he allegedly lied to police about what he was doing on the bridge that night. He told one friend who testified that he was "throwing garbage off." When pressed, he said he would "confess" if enough evidence was gathered, the friend testified.

More than 700 tiny fibers found on Cater, Payne and nine other victims matched material from the three cars he drove, his carpet and bedspread, rugs and a yellow blanket, according to experts. Scientists armed with charts of fiber pinups have testified it would be almost impossible for the victims not to have come into contact with Williams because of their matchups.

To illustrate their theory, an assistant prosecutor, on the courtroom floor, rolled his head across a carpet. Gordon Miller thus demonstrated the "exchange principle"--how fibers are passed from one object to another.

As deputies huddled about Williams, the lights went off and a fiber expert covered Miller in an ultraviolet light. The threads in his hair glowed.

Such fibers are part of a mountain of circumstantial evidence against Williams. No one saw him kill anyone.

A slapjack was found in his ceiling; one victim died from a blow to the head. Like Cater and Payne, most others were strangled or asphyxiated, possibly with a choke hold. Two died from stab wounds; their blood types matched bloodstains in Williams' car.

Alone, that's hardly conclusive, say experts. Stir it together, and the circumstances of guilt are overwhelming, argue prosecutors.

What has emerged from the witness stand beside Judge Clarence Cooper's bench is a portrait in contradiction. Williams cruised the streets in several cars, his radio scanner tuned to police frequencies. Did he use his radio to keep one step ahead of police, or to chase accidents to film as a freelance TV cameraman?

He allegedly lied several times to police, but does that mean he murdered? Those are the kind of doubts the defense attorney is expected to emphasize.