The body of a young black male was found tonight floating on the surface of the Chattahoochee River, several miles from the spot where the 21st victim in Atlanta's string of black child slayings was pulled from the water Monday, police said.

Details were sketchy, but authorities said the body, partly decomposed, was about five feet tall and probably teen-aged. The body, according to Sheriff Earl Lee of Douglas County, was "very sparsely clothed."

The body was taken to the Douglas County morgue, and the special police task force investigating the series of deaths was called to the scene shortly after a canoeist found the body snagged on a tree limb.

If the body is connected to the chain of mysterious Atlanta-area murders, it would be the 22nd black child murder victim.

Earlier today, Timothy Hill became the 21st such victim as medical examiners positively identified the body found floating Monday afternoon in the Chattahoochee as the 13-year-old his mother called "a good little, hard-working boy."

Two other black children remain missing.

Police kept Timothy, missing since March 13, off the official list of missing and murdered children because he had a history of a runaway, and because Atlanta police were still investigating reports that he had been spotted several times by friends.

Police Chief George Napper delivered the news to Timothy's family this afternoon, and politely asked photographers to avoid taking any pictures of a hysterical sister who collapsed and had to be rushed by ambulance to Grady Hospital.

Dr. John Feegel, associate Fulton County medical examiner, who performed the autopsy, identified the partially decomposed body from dental records. But, he said, he could recognize Timothy from photographs, and theorized that the cold river had perserved the body well.

"It's definitely Timothy Hill," Feegel said in an interview. He said he found no marks on the body, and ruled the cause of death as "probable asphyxiation, possibly suffocation."

Eleven of the 23 missing and murdered children who have been haunting this city for the last 20 months have been asphyxiated or strangled. Feegel said he found no signs of mutilation or sexual abuse, although the teen-ager was pulled from underbrush in the river clad only in underwear shorts, also the sole clothing on two other male victims.

Feegel theorized that such lack of clothing could imply a sexual motive to the killings. "We aren't left with any evidence [of sexual abuse]," he said."But when you find a teen-age boy in his underwear, you can construe that there's probably a sexual motive."

Over the last 20 months, 21 children have been found murdered here, all but two of them boys. Darron Glass, 10, has been missing since September; Joseph Bell was reported missing March 3. At 5-feet-5 and 100 pounds, Joseph and Timothy were the same size.

Timothy had a profile similar to those of many of the children who grow up in low-income, single-parent homes on the city's seedy south side, hustling for a buck in the streets, hanging out to learn the ropes of life the hard way.

He palled around with other streetwise boys his age, teen-agers who, even in the shadow of violence, still affect a certain air of invulnerability, brandishing metal pipes and knives for reporters as the weapons they would level against anyone who tried to snatch them.

Other Atlanta children may wet their beds and have nightmares about the elusive killer or killers stalking the city, but not Timothy Hill and his friends, says Terry Nelson, director of the John H. Harland Boys Club in the neighborhood. Timothy and his friends often talked about making "easy money," maybe even catching the killer and pocketing the $100,000 reward, Nelson says.

Two other children on the task force list also frequently the boys club, and one of them, Joseph Bell, 16, lived right down the street from Timothy, whose older brother, Richard, said Timothy also knew Patrick Rogers, 16, whose body was pulled from the Chattahoochee River in December.

Timothy kept to himself, a follower, easily intimidated by other youngsters who threatened him with physical harm, said Nelson.As a child, he was hit by a car.

"Tim was very easily influenced," said Nelson. "He was kind of timid. Some kids would say they'd beat him up, so he'd do anything they wanted."

He was easy prey.

Two weeks after Timothy disappeared, his 14-year-old neighbor was asked where his friend could be found. "He's probably spending the night somewhere," he shrugged.

But Timothy's sister, Brenda, 18, insisted that Timothy was not a runaway. "The police just can't accept the fact that someone came and snatched him," she said last week in a statement that has proved prophetic more than once.

One reason Atlanta police cite for holding Timothy off the official list of missing and murdered children: his special education teacher at Dean Rusk School received two calls recently from a crying child who responded to the name Timothy. Teresa Swindall said she believed the caller was acting so blase about the child killers.

He hungered for money, awed by the shiny Cadillacs that cruised the neighborhood, and wanted that good life, and worked around the neighborhood to earn pocket money. Like many of the other murdered children, he carried grocery bags for spare change. He would do most anything for a dollar.

"He would have gotten into a car with anybody if he thought he could get some money," she said. "He didn't think it could happen to him."