Two police officers in Ferguson, Missouri, were shot and wounded in what officials called an ambush early on Thursday, the latest spasm of violence arising from months of tension between African-Americans and the city's mostly white police force.
The officers were treated and released from hospital, St Louis County Police said. Police Chief Jon Belmar earlier said a manhunt was under way for a suspect or suspects.
A 41-year-old officer from the St. Louis County Police was struck in the shoulder and a 32-year-old officer from the nearby Webster Groves Police Department had a bullet lodged near his ear after it passed through his cheek, Belmar said.
"This is really an ambush, is what it is," Belmar said. "You can't see it coming. You don't understand that it's going to happen."
The shots rang out as a rally in front of the city's police headquarters was dispersing hours after the local police chief resigned. After months of criticism, Police Chief Tom Jackson quit in the wake of a scathing U.S. Justice Department report finding his force was rife with racial bias.
The St. Louis suburb was thrown into turmoil in August by the killing of an unarmed black teenager by a white policeman, thrusting it into the center of an intense national debate over law enforcement's use of deadly force against minorities.
Thursday's shootings were "inexcusable and repugnant," U.S Attorney General Eric Holder said in a statement. "Such senseless acts of violence threaten the very reforms that nonviolent protesters in Ferguson and around the country have been working towards."
Belmar told a news conference authorities had possible leads in finding whoever was responsible but gave no details.
"This is No. 1 priority of St. Louis County police to identify that individual or individuals," said Belmar, who leads the police force in the county that includes Ferguson. Officers did not return fire but may shoot back in future, he said.
"These police officers were ... shot just because they were police officers," Belmar told reporters after the shootings.
"I have said all along that we cannot sustain this forever without problems," he said, referring to festering tensions in the city since the police shooting of Michael Brown last summer.
Benjamin Crump, an attorney representing Brown's parents, said the family condemned the Thursday shootings and insisted a small number of people were responsible for any violence.
"There may be a few people who are misguided or confused but in large part the majority of the protesters and the majority of Americans want justice," Crump told CNN.
Less than three months ago a man shot dead two New York City patrolmen, saying he was seeking to avenge the killings of Brown and an unarmed black man in New York City. In both cases, grand juries decided against bringing criminal charges.
The Ferguson protest started peacefully on Wednesday soon after Police Chief Tom Jackson said he was stepping down, but about two dozen officers in riot gear later faced off with demonstrators and at least two people were taken into custody.
Gunshots rang out about midnight, causing pandemonium. Many of the few dozen demonstrators still present fled, some screaming. Police scrambled, with many taking defensive positions with weapons drawn and some huddling behind riot shields, according to a video published online.
"I don't know who did the shooting, ... but somehow they were embedded in that group of folks," Belmar said.
Protesters at the scene said on social media, however, that the shots did not come from where they were standing.
"The shooter was not with the protesters. The shooter was atop the hill," activist DeRay McKesson said on Twitter.
"I was here. I saw the officer fall. The shot came from at least 500 feet away from the officers," he said.
McKesson said many were not satisfied with the police chief's resignation and wanted the city's mayor, James Knowles, to step down too.
Jackson was the latest in a string of Ferguson officials to resign in the week after the Justice Department report, which found widespread racial bias had led to abuse of African-Americans by the city's police and court.
The investigation found Ferguson used police as a collection agency, issuing traffic citations to black residents to boost city coffers, resulting in a "toxic environment".
After the report, Holder said the federal government would use its full authority to demand police reforms in Ferguson, including possibly dismantling the department.
(Additional reporting by David Bailey in Minneapolis, Fiona Ortiz in Chicago and Carey Gillam in Kansas City; Writing by Frank McGurty; Editing by Tom Heneghan, W Simon, Bill Trott, Bernadette Baum and James Dalgleish)