In what police fear could be a cruel new trend in the child murders here, the special task force investigating the murders of 21 children and one retarded adult today added the name of another retarded young black man to its list of those officially missing.

The case of Larry Rogers, 21, was adopted by the task force this afternoon because of "similarities in his profile and disappearance with some of the other cases," according to a statement released by the Atlanta police.

Rogers, a slightly retarded adult described by his foster father as "a good boy who loved to work and make a little change" cutting grass, raking leaves and cleaning out gutters, disappeared March 30 from his home in Northwest Atlanta. Police say a neighbor watched Rogers climb into a station wagon driven by a black man with long, black hair, a thick, false-looking mustache and false-looking eyelashes. The car sped off, witnesses said.

Another mentally retarded adult, Eddie (Bubba) Duncan, was found last Tuesday in the Chattahoochee River a few miles from the site where 13-year-old Timothy Hill was pulled from the river the day before. Both Hill and Duncan were wearing only undershorts, like two other murdered youths.

Like Duncan, Rogers was not only mentally retarded, he had a childlike appearance and a speech impediment, say police. He was undersized for his age at 5 feet 6 inches and 140 pounds. Police said he was vulnerable, easy prey.

Although the task force took over his case Monday, members of the missing persons unit said they had interviewed two teenagers who claimed to have seen Rogers Sunday across town in DeKalb County watching a softball game. "We showed them a picture of Rogers and they felt certain that he was the one they saw," said one officer.

Police declined comment when asked if the task force was aware of the sighting before they announced adoption of the case. But one official said the task force was taking no chances after recent criticism over its failure to add Timothy Hill to the list until his body was found.

Rogers was only a baby, 15 months old, when county social workers brought him to the modest brick home of retired barber George W. Hood, now 79, and his wife. The Hoods, with no natural children of their own, raised him, along with four other foster children.

When Larry began to walk, they noticed he seemed slow to respond and talk. But special education services were hard to find and they coped as best they could, enrolling Larry in a neighborhood elementary school, recalls Hood. Disadvantaged, with the mental abilities of a 6- or 7-year-old, Larry was promoted because "teachers just got tired of fooling with him," said Hood. "He didn't remember nothing too well."

At 8, the same social workers who had delivered Larry to the Hood home whisked him off to another family. Five years later, the boy, then 13, returned to the Hood household to live permanently.

Rogers was living with his foster father, whose wife died in 1965, when he disappeared March 30. Hood says he didn't report him missing for three days because he didn't "want to make a false alarm." That was last Thursday. d