US President, Barack Obama has called on all Americans to unite and refrain from divisive language in the wake of another gun-related violence in which three police officers were killed on Sunday.

In a live broadcast from the White House, President Obama said that “nothing justifies violence against law enforcement”.

This is coming as more revelations are beginning to emerge about the gunman behind the attack.

The man identified as Gavin Long, a 29-year-old ex US marine had posted videos on YouTube in which he complained about police treatment of African Americans and urging them to “fight back”.

Long, a Marine from August 2005 until August 2010, rose to the rank of sergeant and served in Iraq from June 2008 until January 2009, earning a number of medals and commendations before he received an honourable discharge.

The ex-Marine, who was later killed by police during the attack on Sunday, stressed in one of the video posts that should “anything happen” to him, he was “not affiliated” with any group.

“I’m affiliated with the spirit of justice, nothing more nothing less. I thought my own thoughts, I made my own decisions,” he said.

Tension has been high since the police shot dead a black man in Baton Rouge two weeks ago.

That death – and a second police shooting in Minnesota – sparked protests across the United States and triggered a revenge attack by a black army veteran who shot dead five officers in the city of Dallas.

Sunday’s attack reportedly took place on Long’s birthday. It is unclear why Long was in Baton Rouge and officials say his motives for the attack have not been determined.

The dead officers were named as Montrell Jackson, 32, and Matthew Gerald, 41, of the Baton Rouge police department, and Sheriff’s Deputy Brad Garafola, 45. All three men had families.

It has emerged that just days before the attack Jackson posted an emotional message on Facebook about how hard it was to be a black police officer in Baton Rouge.

“I swear to God I love this city but I wonder if this city loves me,” he wrote.

“In uniform, I get nasty hateful looks, and out of uniform some consider me a threat.”

President Obama said “Everyone right now focus on words and actions that can unite this country rather than divide it further,” he said, as the US begins two weeks of political conventions with Republicans meeting in Cleveland later on Monday.

“We need to temper our words and open our hearts… all of us,” said the president.