Public Safety Commissioner Lee Brown asked the FBI to officially join the investigation of the death of disappearances of 15 black children. After the discovery of the body of the 15th victim -- 4 are still missing -- Mr. Brown said he wants the FBI, which has already assigned two liaison agents and the use of FBI laboratory facilities, to add full-time investigators. The bureau could not enter the case formally except by request because, so far as is known, there has been no violation that would bring the killings and disappearances under federal jurisdiction. Mr. Brown also said the 25-member special police task force will be increased by another 10 officers. In addition, a special patrol will be set up in the neighborhoods where the children disappeared to keep an eye on youngsters, particularly after dark.