S President Barack Obama has called on Americans to "focus on words and actions that can unite this country rather than divide it further" after three police officers were shot dead in Baton Rouge, Louisiana.
The gunman, identified as Gavin Eugene Long, from Kansas City, Missouri, also died at the scene.
The 29-year-old former US marine had served in Iraq before being honourably discharged in 2010 with the rank of sergeant.
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The shooting took place after police responded to a call of a "suspicious person walking down Airline Highway with an assault rifle". It is believed Long made the call himself, reports CNN.
The gunman had posted several videos to YouTube discussing police brutality before the attack and was active on several online forums, where he used the pseudonym "Cosmo Setepenra".
"His history of rambling postings indicated that the attack was motivated at least in part by killings by police of black Americans in recent years and the resulting unrest," says The Guardian. "But they also pointed to apparent paranoia and mental instability."
Long reportedly considered himself a victim of "gang stalking", a form of intense "government and corporate surveillance covering every aspect of a subject's life".
There is a growing national divide over policing in the US. Baton Rouge has been the scene of a number of demonstrations since the fatal shooting of Alton Sterling, a 37-year-old black man, by police on 5 July. Sterling's family has made an emotional plea for peace, saying they don't want any more bloodshed.
Five police officers were also killed in Dallas earlier this month by Micah Johnson, who had served in the US army.
In a live broadcast from the White House, Obama said: "We as a nation have to be loud and clear that nothing justifies violence against law enforcement."
