The gunman who fatally shot five police officers near a protest Thursday night in Dallas said he was motivated by the Black Lives Matter movement, "wanted to kill white people" and had planted explosives around the city, authorities revealed Friday. The suspect — who CBS and the Los Angeles Times identified as 25-year-old Micah X. Johnson — acted alone and was not affiliated with any extremist group. 

"He wanted to kill officers, and he expressed killing white people. He expressed killing white officers. He expressed anger for Black Lives Matter," Dallas Police Chief David Brown told reporters at a news conference just before 9 a.m. EDT. "None of that makes sense. None of that is a legitimate reason to do harm to anyone."

The shooter opened fire in Dallas Thursday night during a protest in honor of the deaths of Philando Castile and Alton Sterling, two black men who were killed by law enforcement this week in Minnesota and Louisiana. ABC News reported there were at least two snipers involved in the attack. The gunman died in a standoff with police. Brown confirmed Friday that three other suspects were in police custody.

Brown said the gunman was motivated by "recent police shootings" and seemed lucid during negotiations. It was the deadliest assault on American police since 9/11.

At the news conference, Dallas Mayor Mike Rawlings called his officers "best in class" and mentioned that they'd completed de-escalation training that had led to the fewest officer-involved shootings than any other large U.S. city. But that wasn't always the case. Dallas has a history with police brutality and protests.

In 2012, an officer fatally shot an unarmed black man, James Harper, during a foot chase after receiving a fake call about a kidnapping. The incident brought out hundreds of protesters, some of whom accused the police of a coverup. A grand jury later chose not to indict the officer in question.

A report released last year by the Better Government Association found that Dallas, the ninth biggest city in the country, had the third-highest rate of deadly police shootings in a selection of 10 large cities between 2010 and 2014, according to the Texas Observer. In 2009, 147 people filed complaints against officers alleging they'd used excessive force, the Dallas Morning News reported.

This number dropped to about 13 last year after the training and the adoption of body cameras. However, officers resorted to using force — think baton strikes, pepper spray and Tasers — 2,200 times, according to the News

Brown has been accused of mismanaging the department amid climbing numbers of violent crimes. Last September, Rochelle Bilal, the vice chair of the National Black Police Association, declared that "the Dallas Police Department is broken," according to the News. A Change.org petition calling for the Justice Department to investigate the department garnered about 1,300 signatures but was ultimately closed.