D.C. police are searching for a 33-year-old janitor in the slaying of WMAL radio advertising saleswomen Kathleen Boyden, who was bludgeoned to death in her fashionalbe Northwest Washington apartment last Thursday night.

David I. Garris, of 1812 Vernon St. NW, was still at large late yesterday, about 24 hours after police obtained a warrant charging him with murder.

After an intensive, four-day search, police found Boyden's nude body in the back of her 1980 Toyota shortly after 1 p.m. Tuesday in the 3400 block of 17th Street NW in Mount Pleasant.

Police said yesterday that Garris, a janitor in Boyden's apartment building at 3040 Idaho Ave. NW had been interviewed there several times by detectives while Boyden was still considered a missing person. They said, however, that they had no evidence to justify issuance of a warrant until late Tuesday night.

According to Lt. William Ritchie, Boyden was slain some time before dawn Thursday. He said there were no signs of forced entry into Boyden's apartment and police believe the killer has a key.

Police investigators were uncertain yesterday whether Boyden had returned home late and surprised the killer or whether her assailant entered the apartment after she had gone to sleep.

Police said Garris was convicted of rape several years ago but that the conviction was overturned on appeal. They also said Garris served time in prison after he was convicted of violating the Mann Act -- transporting females across state lines for prostitution or other immoral purposes.