The black former Marine and Iraq war veteran who shot dead three police officers in the southern US city of Baton Rouge deliberately targeted the cops, but his motive remained unclear, officials said Monday.

Louisiana investigators called for patience as they determine why the shooter -- identified as Gavin Long -- opened fire on the officers on Sunday, the day he turned 29. He also wounded three police officers before dying in a firefight.

It was the second calculated attack on law enforcement in 10 days, after the slaying of five officers in Dallas -- attacks that have kept the nation on edge.

It was not yet clear if Long had similar motivations as Micah Johnson, a black veteran of the Afghanistan war who opened fire in Dallas to avenge the fatal police shootings of black men, with white cops his stated target.

But like Johnson, Long was a heavily-armed black man whose social media posts suggest he was incensed by racial bias towards African Americans.

"There's no doubt whatsoever these officers were intentionally targeted and assassinated," Louisiana State Police Superintendent Colonel Mike Edmonson told a news conference.

"He was not going to stop here," added Baton Rouge police chief Carl Dabadie. "After he was finished here, I have no doubt he was heading to our headquarters and he was going to take more lives."

- 'Landscape has changed' -

Racial tensions were already high in Louisiana's state capital after the July 5 death of Alton Sterling, a black man shot at point-blank range by white police.

Police had yet to establish a connection between Sterling's death and Sunday's attack, however.

"We haven't ruled out anything, but there's nothing to believe that we can say it has something to do with that," Edmonson said.

"Clearly the landscape has changed since Dallas," he added.

Around 200 people gathered Monday night at a church in Baton Rouge for a vigil for Matthew Gerald, one of the slain officers who was also a former soldier.

"We are here to support his mother," said Linda Furr, head of the local chapter of the Blue Star Moms, which brings together mothers with sons or daughters serving in the military.

President Barack Obama has condemned the "cowardly" Baton Rouge shooting, demanding an end to violence against law enforcement and renewing his plea for national unity.

"It is so important that everyone... right now focus on words and actions that can unite this country rather than divide it further," he told reporters at the White House.

The shooting of Sterling and another by Minnesota police of a black motorist led to a nationwide wave of marches against police abuses. The dying moments of both men were captured in video footage that went viral online.

The murder of five Dallas officers came during a protest against police brutality. Johnson told negotiators he wanted to kill white cops -- before he was killed by a police robot bomb.

- Troubled shooter -

The Baton Rouge shooting took place along a highway near the police headquarters on Sunday at around 8:40 am (1340 GMT), after officers responded to a call about a man carrying a rifle.

Police showed security camera footage of the suspect, dressed all in black and wearing a mask, in the moments before he "ambushed" the officers, in Edmonson's words.

Officials said he was armed with an IWI SAR assault rifle and a Springfield handgun. A military-style Stags Arm assault rifle was also recovered in his vehicle.

He was taken out by a police SWAT team rifleman, from more than 100 yards (meters) away.

Dabadie said the shooting was evidence of the need for a well-armed police force, even as US law enforcement methods come under scrutiny.

"We've been questioned for the last three or four weeks about our militarized tactics and our militarized law enforcement. This is why. Because we are up against a force that is not playing by the rules," Dabadie told the news conference.

- 'Conspiracy theories' -

Based in Kansas City -- more than 700 miles (1,100 kilometers) north of Baton Rouge -- Long was a former Marine whose served a 2008-2009 tour of duty in Iraq.

Edmonson said the gunman had been in Baton Rouge for several days, apparently "looking for locations to specifically target police officers."

In 2015, Long legally changed his name to Cosmo Ausar Setepenra, claiming to be a member of the Washitaw Nation, a group of African-Americans claiming to be a Native American nation in the United States.

According to SITE, a group that monitors extremist movements, he "ascribed to multiple fringe movements and conspiracy theories online, and expressed an unwillingness toward 'peaceful protests'."

Edmonson said investigators were working hard to examine the gunman's social media footprint -- declining to confirm the authenticity of a Twitter feed widely attributed to Setepenra.

That account -- on which Setepenra said he traveled to Dallas after the July 7 shooting -- was filled with posts directed against white people.

But one of the officers killed by Long -- Montrell Jackson, 32 -- was black. The other two were identified as Gerald, and Brad Garafola.

"I swear to God I love this city but I wonder if this city loves me. In uniform I get nasty hateful looks and out of uniform some consider me a threat," Jackson wrote recently on Facebook.