Two snipers shot and killed four Dallas city police officers and a Dallas Area Rapid Transit (DART) officer Thursday night during a Black Lives Matter protest downtown. Six other officers were wounded in the coordinated attack.

A man who exchanged gunfire with police in the El Centro College garage was reported dead shortly before 3 a.m., the Dallas Morning News reports.

The shooting was the deadliest day for law officers since Sept. 11, 2001, when 72 officers died, according to the National Law Enforcement Officers Memorial Fund.  

Dallas Police Chief David Brown said snipers with rifles shot 11 officers and one bystander from elevated positions about 9 p.m..

The names of the Dallas police officers have not been released, but DART identified the officer it lost as 43-year-old Brent Thompson, who joined the department in 2009.

Thompson is the first officer to be killed in the line of duty since DART formed a police department in 1989, spokesman Morgan Lyons said.

Three Dallas police officers were in critical condition, and two of them had undergone surgery, police said. 

The officers were assaulted "ambush-style," Brown said, with some of them shot in the back.

Brown said it's unclear how many suspects were involved, but three people are in custody. At least one turned himself in voluntarily.