Five police officers were killed and seven others wounded in a sniper-style attack by a black army veteran during a protest in Dallas against police brutality towards African-Americans.

Two civilians were also wounded on what was said to be the single deadliest day for law enforcement since September 11, 2011.

Here's what we know so far:

What happened?

Hundreds of demonstrators had gathered Thursday in Dallas, Texas, a city of 1.2 million people, to protest over the fatal police shootings of two black men this week in Louisiana and Minnesota.

The shootings revived an emotional debate over lethal use of force by police, and problems of alleged police racial bias targeting minorities, especially African-Americans.

At roughly 9:00 pm, as the rally was winding up, shots rang out.

At least one shooter took aim at police. The shooting occurred not far from Dealey Plaza, where president John F. Kennedy was assassinated in 1963.

What was the death toll?In all, five officers were killed, including one Dallas transit cop. Seven others were wounded, along with two civilians.

Police cornered the main shooting suspect, and tried to negotiate with him, but the talks broke down after several hours.

There was an exchange of gunfire with police, and the suspect finally died in a blast caused by a robot-controlled explosive device sent in by officers.

"He threatened other bombs, and we felt that was the safest way to get in and it was," Dallas Mayor Mike Rawlings said.

Who is behind the attack?

Police formally identified the slain suspect as Micah Johnson, 25. The black man, a resident of the Dallas suburb of Mesquite, had no prior criminal record and was described to police as a "loner."

The US Army confirmed that Johnson served as a reservist for six years, including a tour of duty in Afghanistan.

Bomb-making materials, weapons and ammunition were found in his home, police said.

Police initially said it seemed two shooters were involved, but US Homeland Security Secretary Jeh Johnson said Friday the gunman appeared to have acted alone.

However, Texas Governor Greg Abbott said police would "continue down every rabbit trail... ensuring that we eliminate any other possible suspects or co-conspirators who may have aided this gunman in any way."

What was the motive?

Dallas police chief David Brown said that in negotiations with police during the ultimately fatal standoff, the gunman said he was "upset about the recent police shootings."

"The suspect said he was upset at white people. The suspect stated he wanted to kill white people, especially white officers," Brown said.

While the White House ruled out any link between the gunman and known "terrorist organizations," Johnson's Facebook page ties him to several radical black activist movements listed as hate groups by the Southern Poverty Law Center.

Groups that he "liked" include the New Black Panther Party (NBPP) and the Nation of Islam, both known for expressing virulently anti-Semitic and anti-white views, the SPLC said in a statement.