Mobs of protesters smashed storefronts, lobbed bottles and set fires overnight in Oakland, Calif., hours after a white transit officer was found guilty of involuntary manslaughter in the shooting death of an unarmed black man on Jan. 1, 2009. Dozens of arrests were made late Thursday and early Friday, according to The Associated Press, as the police used riot gear and a heavy presence to try to disperse the free-ranging crowd, which ransacked stores and stampeded through city streets whenever officers surged to make arrests. City officials were worried about a reprise of the riots that erupted in downtown Oakland after the shooting death last year of Oscar Grant III, when crowds burned cars and smashed storefronts. Mr. Grant’s shooting was captured on cellphone video and widely disseminated on the Internet. The verdict also prompted Justice Department officials to announce Friday that they would look into whether the case warranted federal prosecution. After the verdict was announced Thursday, several hundred people gathered near Oakland City Hall and taunted and threw bottles at police officers in riot gear. At least one person was either hit by a vehicle or injured by the surging crowd. Hundreds of others listened peacefully to speakers who had gathered downtown.