''It is unknown whether he was followed at this time or not,'' he said. ''I think that's a good probability. But it may have been a chance opportunity that somebody took, robbery being a possible motive.''

Mr. Cosby was killed shortly before 2 A.M., the police said. He was driving on the city's western side, headed north on Interstate 405 through Sepulveda Pass, when the tire went flat.

Family friends said Mr. Cosby might have been going to visit one of his many friends in the San Fernando Valley. They described him as a down-to-earth young man, little affected by his father's fame, and they said that while visiting Los Angeles he liked to drop in on his friends for late-night talks.

Mr. Cosby's father and his mother, Camille live in Manhattan and were notified of their son's death in a call from Commander McBride. Having lost a 16-year-old daughter in a hit-and-run accident, the officer spoke with the couple for an additional 20 minutes about coping with the death of a child.

Bill Cosby, whose current hit television show is titled ''Cosby,'' then briefly spoke with reporters outside his apartment, calling Ennis ''my hero'' and then, his eyes puffy, his face drawn, returning inside.

Mr. Cosby's comedic stock in trade -- for eight years on ''The Cosby Show'' -- was on-screen interaction and antics between him and members of his television family, particularly the children. Much of that material, as well as his stand-up appearances and several books, was undoubtedly drawn from life with his five real-life children. Among his books that touch on child rearing are ''Fatherhood,'' ''Time Flies'' and ''Childhood.''

Later today he and Mrs. Cosby issued this brief statement through his office: ''We have every confidence in the Los Angeles Police Department. Our hearts go out to each and everybody who has suffered a loss like this. This is a life experience that is truly difficult to share.''