The mother of the 5-year-old boy who was found decapitated, dismembered and disemboweled on a love seat in a modest Prince George's County apartment late Sunday was charged last night with first- degree murder, police spokesman Robert Law said.

Law said that Erica Mendell Daye, 25, was in police custody after being located at St. Elizabeths Hospital in Southeast Washington, where she was a patient.

Law said police have no motive for the killing, which investigators described as "brutal and sadistic" and unprecedented in the county.

Daye and her son, William Lawrence Deloach, had been living in the University Garden Apartments complex at 7950 18th Ave., Adelphi, since August. The management office at the complex declined to answer questions yesterday about the mother and her son.

Law said Daye was found at St. Elizabeths, a federal mental institution, after a nurse there saw an evening television news report featuring Daye's picture, which was released by police trying to locate her for questioning in the case.

"The woman turned herself in to D.C. General Hospital this afternoon, and the hospital transferred her to St. Elizabeths," Law said.

Daye is charged in the District of Columbia with being a fugitive and could be the subject of extradition proceedings to return her to Prince George's to face the murder charge. She is scheduled to have a hearing today in D.C. Superior Court.

Robert Manley of 7961 18th Ave., Adelphi, a neighbor who said he knows Daye, described her as "a wild type of girl . . . . She messed with drugs."

Decapitation was listed as the cause of death by the Maryland medical examiner's office. Police said the boy was alive when he was decapitated sometime during the 24 hours preceding the discovery of his body at 5 p.m. Sunday, but officials declined to say what instrument was used in the killing.

Up to 20 officers were assigned to the case. Daye had been missing since her son's body was found.

Police have talked to the boy's father, whose name has not been released, and to other family members and friends in the hope of learning what happened and why, Law said.

The killing was reported to police by Daye's sister, who had gone to the apartment to pick up Christmas gifts, Law said. The sister had a key to the apartment, he said. When she found the boy, whose head and arms had been cut off, she notified police, who described her as "very, very hysterical."

Homicide Detective Garland Price said that county police responded to a call from Daye's telephone Saturday between 3 and 4 p.m. The person who called reported a suspicious man with a rifle in the area, Price said. He said two officers searched the area around 18th Avenue but found no suspicious man.

When police went to Daye's second floor apartment to pursue the matter, Price said, a woman said from behind a closed door that she did not need police assistance.

Police said they are not sure if Daye placed the phone call and if she was the woman who talked to officers from behind the door.

The slaying has frightened many residents in the neighborhood where the boy and his mother lived.

Some neighbors refused to answer knocks on their doors yesterday. One woman, who lives across the street from Daye's apartment, waited outside yesterday for the school bus transporting her 4-year-old daughter home.

"Of course I'm scared," said Linda Parsons, 24, who lives in a town house in the 7900 block of 18th Avenue. "We don't know what happened in there."