The troubled US community of Ferguson, Missouri - scene of months of protests and racial tension - is on a knife edge again after two police officers were shot.
One officer was shot in the face, and the other in the shoulder, after the city's embattled police chief resigned.
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."
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.
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