Dallas - Five police officers were killed and seven others wounded in a sniper-style attack in Dallas during a protest against police brutality in the wake of two fatal shootings of black men in other US cities.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 in Dallas, as in several other US cities, to protest over the fatal police shootings of two black men this week in Louisiana and Minnesota.The controversial shootings revived an emotional debate in the United States over lethal use of force by police, and problems of alleged police racial bias targeting minorities, especially African-Americans.At roughly 21:00, as the rally was winding up, shots rang out.At least one shooter took aim at police.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.Where did it happen?Dallas is one of the biggest cities in the state of Texas, with a population of roughly 1.2 million people.The shooting took far not far from Dealey Plaza, the site where late president John F Kennedy was assassinated in 1963.Who is behind the attack?US media have identified the slain suspect as Micah Xavier Johnson, 25, of the Dallas suburb of Mesquite.Police have so far offered little information about the shooter, citing the needs of their investigation.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.Police initially said three other suspects are in custody, but were more vague about the number on Friday, refusing to offer a figure.