OAKLAND, Calif.  Even as city residents cleaned up after a night of looting and vandalism, Mayor Ron Dellums of Oakland said he supported a federal investigation that is under way on the death of Oscar Grant III, a young unarmed black man who was shot and killed by a white transit police officer.

The officer, Johannes Mehserle, was convicted of involuntary manslaughter on Thursday by a Los Angeles jury that rejected a more serious charge of second-degree murder. Mr. Grant, 22, was shot in the back on Jan. 1, 2009, while lying face down on a subway platform; in his defense, Mr. Mehserle said he had meant to draw and use his Taser, not his gun.

The verdict, which came after a little more than a day’s deliberation, was met with both peaceful protests and more violent skirmishes after dark, with the police making 78 arrests and dodging homemade bombs, rocks and other projectiles. But only three injuries were reported, one to a civilian and two to the police.

On Friday, Mr. Dellums praised his police department’s performance and said he would back an investigation by the Justice Department and the F.B.I. The Justice Department said it was looking “to determine whether the evidence warrants federal prosecution.”