FREDERICK, Md. (AP) _ A 9-year-old boy whose naked body was found in a baseball field dugout was beaten, strangled and sexually assaulted, police said Wednesday.

A mentally retarded homeless man with a history of sexual offenses was charged with first-degree murder in the death of Christopher Ausherman. The suspect, Elmer Spencer Jr., had been released from prison less than a week before the killing.

Spencer, 44, was arrested at a homeless shelter Monday night. He had been released from prison after serving about four years of a 10-year sentence for assaulting a woman and escaping from the arresting officer.

Spencer, who was on probation, had served two previous prison terms for child sexual assault and battery convictions, according to court documents. His record also includes charges of sexually assaulting a 7-year-old girl in 1996 and assaulting, with intent to murder, an 8-year-old boy in 1995. Both cases were dropped.

Leonard Sipes, a spokesman for the state corrections department, said Spencer was released early from his latest prison term because of good behavior credits accrued over the 13 years he served on a 22-year sentence for sexually assaulting a child in 1981.

Police said Spencer was tracked down through an acquaintance who identified him as the man seen with the boy in surveillance videotapes taken at two convenience stores.