A body found yesterday in a river in southeast DeKalb County, Ga., was identified last night as that of 15-year-old Joseph Bell, one of Atlanta's missing black children.

DeKalb County public safety director Dick Hand said that medical examiners identified the body as Bell, who was last seen more than a month ago at a restaurant where he occasionally did odd jobs, but details of the identification process which used dental records were not immediately available.

Hand said the cause of death was not immediately known, and that an autopsy would be performed today in an effort to determine how Bell died. The body was spotted in the South River by a couple trying out a motorbike.

An unnamed source told the Atlanta Constitution that the body was clad only in undershorts, as were four of the last five victims.

According to Hand, Bell's death appeared to be a homicide. The body, he said, had been in the water for an "extended period of time," probably about a month.

If an autopsy shows Bell's death to have been the result of homicide, he would be the 24 young black to be found dead in the Atlanta area since July 1979 in a series of murders that have triggered a national outpouring of grief and concern, as well as money to press the often-stymied investigation. A 25th child, 10-year-old Darron Glass, who disappeared Sept. 14, is still listed as missing.

Six of the dead black youths have been found in Atlanta area rivers. Three, including Bell, were found in the South River. Three have been found west of Atlanta in the Chattahooche River.

Most of the murdered youngsters have been in their middle teens or younger, but the list includes two retarded men, both in their 20s.

Firefighters recovered Bell's partly decomposed body late yesterday afternoon from a dry area where the river had receded at the boundary between DeKalb and Rockdale counties, and the special task force formed to investigate the killings immediately rushed to the scene. DeKalb County police spokesman Chuck Johnson said that the investigation of the death will be conducted jointly by the DeKalb County officials and the task force.

The incidents have spawned vigilante groups in poor Atlanta neighborhoods, where many of the young victims lived. Throughout it all, there have been a number of leads in the cases, but none are known to have brought the 31 FBI investigators and nearly 35 Georgia investigators closer to a solution.

The latest victim, Joseph (Jo-Jo) Bell, was a slim, street-wise teen-ager who was last seen March 2 at a seafood carry-out restaurant in Atlanta. The next day, said owner Richard Harp, someone phoned the restaurant, identified himself as Jo-Jo, and said, "I'm almost dead," and hung up.

However, because the youngster had a history of running away, he was not listed as missing until March 13. Harp said the youngster in the face of the killings. "He said no kid snatcher was going to get him," said Harp.

Meanwhile, Atlanta area newspapers reported yesterday that investigators were questioning several people who resemble the composite sketch of a man seen with one of the missing or murdered children.

That victim was identified as Larry Rogers, 21, whose body was found earlier this month in an abandoned apartment building. He reportedly was seen entering a faded, 1966 green Chevrolet station wagon on March 30, when he was last seen alive.

United Press International quoted "a source close to the investigation" as saying that investigators have "got some pretty good people to look at" who resemble the composite of the man whose car Rogers supposedly entered.

"The police have picked up several people and I don't know that they've been eliminated," UPI quoted its source as saying. "The feedback I'm getting is that they're real good ones and they're looking at them real close. But we have to have something more than we have to call them suspects."

In Atlanta Easter sunrise service yesterday, held at the gravesites of six of the murder victims, the Rev. Arthur Langford prayed that the tragedy would end soon. He also urged worshippers to keep closer watch over their children.

"A message has come out of all of this that we have a job to do," Langford said. "We must strengthen the institution of the family. We must protect our children. We must love them," he said.