Milwaukee - The black man whose killing by US police touched off rioting in Milwaukee was shot by a black officer after turning toward him with a gun in his hand, the police chief said on Sunday.Wisconsin's governor put the National Guard on standby in case of a repeat of the violence that rocked the city earlier in the weekend, but after dark on Sunday protests were so far peaceful.Police Chief Edward Flynn cautioned that the shooting was still under investigation and authorities were awaiting autopsy results, but that based on the silent video from the unidentified officer's body camera, he "certainly appeared to be within lawful bounds".At the same news conference, Mayor Tom Barrett said a still image pulled from the footage clearly showed the gun in 23-year-old Sylville K. Smith's hand as he fled a traffic stop on Saturday."I want our community to know that," Barrett said. But he also called for understanding for Smith's family."A young man lost his life yesterday afternoon," the mayor said. "And no matter what the circumstances are, his family has to be hurting."'Behaving suspiciously'Flynn declined to identify the officer who shot Smith but said he is black. The police chief said he wasn't sure what prompted the stop but described Smith's car as "behaving suspiciously".After watching the officer's body camera footage, Flynn said the entire episode took about 25 seconds, from the start of the traffic stop until shots were fired. He said Smith ran "a few dozen feet" and turned toward the officer while holding a gun."It was in his hand. He was raising up with it," the chief said, adding the officer had told Smith to drop the gun and he did not do so. It was unclear how many rounds the officer fired. Smith was hit in the chest and arm, Flynn said.Governor Scott Walker activated Wisconsin's National Guard, and 125 Guard members were reporting to local armouries to prepare for further instructions. Flynn said they would not be deployed unless the chief decided to do so. Flynn said 150 department officers specially trained in managing big protests had also been mobilised.Six businesses were burned in the unrest that spilled past midnight on Sunday. Seventeen people were arrested, Flynn said, and four officers were hurt from flying concrete and glass, although all of them had been released from hospital.After dark on Sunday, about a hundred protesters marched to a police station, shouting "Indict. Convict. Send the killer cops to jail," and "Who's streets? Our streets." Police were lined up in riot gear behind a row of squad cars. The protesters were shouting at police but there had been no incidents.'Warning cry'Milwaukee Alderman Khalif Rainey, who represents the neighbourhood that erupted, said the city's black residents are "tired of living under this oppression.""Now this is a warning cry. Where do we go from here? Where do we go as a community from here?" he asked.Milwaukee County Sheriff David Clarke said Smith had been arrested 13 times. Online court records showed a range of charges against Smith, many of them misdemeanours.The Milwaukee Journal Sentinel reported that Smith was also charged in a shooting and was later charged with pressuring the victim to withdraw testimony that identified Smith as the gunman. The charges were dropped because the victim recanted the identification and failed to appear in court, Chief Deputy District Attorney Kent Lovern told the newspaper.On Sunday morning, about three dozen volunteers swept up glass and filled trash bags with rocks, bricks and bottles at the intersection where a petrol station burned to the ground. One volunteer picked up a bullet casing and handed it to police.Darlene Rose, 31, said that she understands the anger that fuelled the violence, but that it doesn't help."I feel like if you're going to make a difference, it's got to be an organised difference," Rose said. "The people that came and looted, you're not going to see them here today."Smith's sister told The Associated Press that the family wants prosecutors to charge the officer who shot him.Kimberly Neal, 24, spoke as supporters surrounded her at the vigil as she held a bouquet of blue balloons.She asked people for donations for his burial.Asked about the violence on Saturday night, Neal said: "People stuck together and they are trying to stand up," for their rights.Tension across USThe anger at Milwaukee police is not new and comes as tension between black communities and law enforcement has ramped up across the nation, resulting in protests and the recent ambush killings of eight officers in Baton Rouge, Louisiana, and Dallas.Nearly 40% of Milwaukee's 600 000 residents are black, and they are heavily concentrated on the north side.Milwaukee was beset by protests and calls for police reform after an officer shot and killed Dontre Hamilton, a mentally ill black man, in 2014.In December, the US Justice Department announced it would work with Milwaukee police on changes.Critics said the police department should have been subjected to a full Justice Department investigation like the one done in Ferguson, Missouri, after the killing of black 18-year-old Michael Brown in 2014 touched off violence there.The officer involved in the most recent Milwaukee shooting was 24 years old and has been an officer for three years, according to the department.At one point on Saturday evening, as many as 100 protesters massed at 44th Street and Auer Avenue, surging against a line of 20 to 30 officers. The Journal Sentinel reported that some in the crowd smashed a squad car's windows. Another police car was set on fire.In addition to the gas station and auto parts store, a bank and a beauty supply shop were burned. Firefighters held back from the gas station blaze because of gunfire.