Chief Daniel Hahn of the Sacramento Police Department said he welcomed the state’s involvement in the wake of “extremely high emotions, anger and hurt in our city.” He said his department would grant full access to state investigators.

Image Stephon Clark Credit... Renee C. Byer/The Sacramento Bee

Mr. Clark was killed March 18 by officers responding to a report of a suspect breaking the windows of vehicles. Officers confronted Mr. Clark in the backyard of his grandmother’s home, where he lived, at which point he advanced toward the officers while extending an object, according to an initial police statement.

Video from body cameras and a police helicopter overhead showed that officers shouted that Mr. Clark had a gun, and then fired 20 shots, continuing to shoot after he dropped to his hands and knees.

When officers examined Mr. Clark’s body, however, the only object they found was a cellphone.

Benjamin Crump, a prominent civil rights lawyer retained by Mr. Clark’s family and who earlier represented the family of Trayvon Martin of Florida, called on Monday for “justice” for Mr. Clark at a public appearance with Sacramento civil rights leaders.

Mr. Becerra promised that the state, working with the local police, would pursue a thorough and fair investigation.