Prosecutors charged a Milwaukee police officer Thursday with killing a black man in August, alleging the man had thrown his gun away and was unarmed when the officer fired the fatal shot.

Dominique Heaggan-Brown, who is also black, was charged with reckless homicide in the Aug. 13 death of Sylville Smith, which sparked two days of riots on Milwaukee’s north side. In the days after the shooting, both the police chief and the mayor had said that police video clearly showed Smith had a gun and was turning toward officers when he was shot. Thursday’s criminal complaint echoed that, but went on to describe a second shot, fired into Smith’s chest after Smith no longer had his gun.

Police Chief Edward Flynn called the charge ‘‘a little difficult to understand’’ and said he hadn’t seen any obvious wrongdoing by Heaggan-Brown in the footage.

Heaggan-Brown, who was fired in October over an unrelated sexual-assault case, shot Smith following a traffic stop. After fleeing police, Smith turned with a gun and was shot once in his biceps, according to the complaint. The second shot occurred less than two seconds later, after Smith was lying on the ground with his hands near his head, according to the complaint.

Milwaukee County District Attorney John Chisholm said in the complaint that the video shows Smith throwing the gun over a fence after the first shot. Heaggan-Brown told state agents that he believed Smith’s gun ‘‘flew’’ out of his hand over a fence after the first shot. The officer said he thought Smith was reaching for another weapon so he fired the second shot.

Chisholm did not take any questions from reporters Thursday. His office said the video would not be released.

Heaggan-Brown was scheduled to make an initial court appearance Friday. His attorney, Jonathan Smith, said that he hasn’t seen any of the state’s evidence but a read of the criminal complaint raises ‘‘issues.’’ He didn’t elaborate but promised a ‘‘vigorous’’ defense.

Smith’s family issued a statement thanking Chisholm.

‘‘We appreciate that the District Attorney has shown independence and sound judgment in prosecuting the officer who shot and killed Sylville,’’ the statement said.

His mother, Mildred Haynes, told the Milwaukee Journal Sentinel the charge should have been tougher.

‘‘He shot him in the arm and shot him again in the chest. . . . To me, he shot to kill,’’ she said.

Flynn said on the day after the shooting that Heaggan-Brown opened fire after Smith turned toward the officer and began to raise his gun despite warnings to drop it. The chief did not mention that Smith was unarmed when the second shot was fired and said it appeared to him the shooting was lawful.

Flynn reiterated that stance Thursday. He called the confrontation a ‘‘combat situation’’ and said the brain needs time to tell the finger to stop pulling the trigger. He said he hopes Chisholm has more evidence that hasn’t been released because he doesn’t want his officers to face deadly offenders and get no allowance for stress.