FERGUSON, United States (AFP) - The troubled US community of Ferguson, Missouri - scene of months of protests and racial tension - was on a knife edge again Thursday after two police officers were shot.
A Swat team hunting for suspects raided a brick bungalow about four blocks from where the shooting happened overnight.
Neighbours said three people had been taken away, including a woman in handcuffs.
President Barack Obama and Attorney General Eric Holder were quick to denounce the "ambush," which threatened to derail efforts to calm nerves in the mainly black town.
Last year, rioting erupted in Ferguson and protests spread to several US cities after a white police officer shot dead an unarmed black teenager.
Then last week, Holder's Justice Department said investigators had found evidence of deep-seated institutional racism in the city's government and in the overwhelmingly white police force.
Ferguson leaders have vowed to reform and several have quit, but officials expressed concern that the latest attack - in which one of the officers was shot in the face - will only deepen the bitter divide in the community.
"What happened last night was a pure ambush," Holder said.
"This was not someone trying to bring healing to Ferguson, this was a damn punk, a punk who was trying to sow discord."
Obama, who on Saturday marked the anniversary of the Selma civil rights march with a major address on the United States' recent history of racism, was also angered.
"Violence against police is unacceptable. Our prayers are with the officers," he wrote on Twitter, warning that the "path to justice is one all of us must travel together."
'BY GOD'S GRACE'
The family of Michael Brown, the young man whose death last year in a police shooting triggered months of protests, condemned the "senseless shooting" and appealed for calm.
"We specifically denounce the actions of stand-alone agitators who unsuccessfully attempt to derail the otherwise peaceful and non-violent movement that has emerged throughout this nation to confront police brutality," they said.
Overnight, one officer was shot in the face and the other in the shoulder amid protests outside the Ferguson police station to mark the resignation of the embattled police chief.
At least three shots were fired, witnesses said.
"By God's grace, we didn't lose two officers last night," said St Louis County police chief Jon Belmar, as detectives combed the scene.
"This is really an ambush." Belmar told reporters.
"That is something that is very difficult to guard against when you have a group of officers standing together in a large group and you have gunfire, certainly gunfire directed at them."
The shooting came just hours after police chief Thomas Jackson resigned, in the wake of a blistering Justice Department report that uncovered racial bias among his officers.
Protest leaders expressed doubt that the shooter, who is thought to have been armed with a handgun, was among their ranks.
The injured officers - one, aged 41, from the St. Louis County force, and the other, 32, from the St Louis suburb of Webster Groves - have been released from hospital, the St Louis Post-Dispatch reported.
One still had a bullet lodged behind his ear, Belmar said.
Wednesday was the 215th night of demonstrations in Ferguson - some small, some huge - prompted by the Aug 9 shooting of Brown by officer Darren Wilson on a residential street.
Wilson says the 18-year-old tried to grab his weapon, a view accepted by a grand jury and the Justice Department report.
Others contend that Brown had put his hands up in a gesture of surrender when Wilson, who has left the Ferguson police force, opened fire.
Describing Wednesday night's incident, Belmar said officers saw "muzzle flashes" and estimated the shooter was perhaps 125 yards away.
They drew their weapons, but did not fire back into the darkness, he said.
"When the shooting rang out, it took everybody by surprise," Lawrence Bryant, a photographer for the St Louis American newspaper, told CNN.
The protest had been generally peaceful, with only two people arrested at different moments for blocking the street, Belmar said.