BATON ROUGE • The authorities have sought to learn more about a former United States Marine sergeant who slew three police officers in Baton Rouge, some two weeks after police there shot dead a black man, sparking nationwide protests including one shattered by the killing of five Dallas policemen.

The suspect, dressed in black and armed with a rifle, was shot dead on Sunday morning in a gunfight with police who converged on the scene of a confrontation that Mayor Kip Holden said began as an "ambush-style" attack.

Two Baton Rouge Police Department officers and one sheriff's deputy were killed, and one sheriff's deputy was critically wounded. Another officer and one other deputy suffered less severe wounds.

A source, speaking on condition of anonymity, said an emergency 911 call may have been used to lure police to the scene where the attack would happen.

Superintendent of Louisiana State Police Michael Edmonson said several officers came under fire as police responded to a report of a man dressed in black standing behind a store holding a rifle.

In the ensuing pandemonium caught on a recording of emergency radio traffic, police repeatedly reported "officer down" and "deputy down" as officers swarmed the area, ultimately confronting the gunman. The episode was over in about eight minutes.

Killed were Mr Montrell Jackson, 32, Mr Matthew Gerald, 41, and sheriff's deputy Brad Garafola, 45.

Colonel Edmonson told a news conference the gunman was believed to have acted alone.

It was not immediately clear whether there was a link between the bloodshed and unrest over the police killings of two black men in questionable circumstances this month - Mr Alton Sterling, 37, in Baton Rouge on July 5, and Mr Philando Castile, 32, near St Paul, Minnesota, on July 6.

Police did not identify the suspect, but a US government official told Reuters he was Gavin Long of Kansas City, Missouri. Long, who was black, was reported to be 29 years old.

Col Edmonson told the Associated Press yesterday the gunman was "certainly seeking out police" in Louisiana's capital, a city with a long history of distrust between African-Americans and law enforcement.

Last week, police arrested more than 100 protesters taking part in a demonstration against police brutality in Baton Rouge under the banner of the Black Lives Matter movement.

"Until we come together as a nation, as a people, to heal as a people, if we don't do that and this madness continues, we will surely perish as a people," a shaken Col Edmonson said at the press conference.

President Barack Obama condemned the attack, vowing justice would be done and calling on Americans to focus on unity.

"We as a nation have to be loud and clear that nothing justifies violence on law enforcement," Mr Obama said in televised remarks.

"We need to temper our words and open our hearts, all of us."

Mr Obama has sought to balance concerns about police abuses, largely against African-Americans, while paying tribute to fallen officers.

He attended a service last week for the five Dallas police officers killed by a black former soldier who opened fire at the end of a protest on July 7, and also denounced the Sterling and Castile slayings.

Attorney-General Loretta Lynch yesterday condemned the Baton Rouge attack and said agents from the Federal Bureau of Investigation and other federal agencies were in the city to help the local authorities.

REUTERS, AGENCE FRANCE-PRESSE