WASHINGTON — A motorcyclist who was fatally shot by a District of Columbia police officer bled heavily on the sidewalk next to a police car as an onlooker screamed and officers performed CPR on the wounded man for more than three minutes, a video released Tuesday shows.

Terrence Sterling, 31, of Fort Washington, Maryland, was shot by an officer in the early morning of Sept. 11. Police said Sterling, who was black, was shot after he rammed the passenger-side door of a police car while trying to flee a traffic stop. His death has prompted protests from people who knew him and activists seeking transparency about what led to his death.

The officer who shot Sterling — identified by police Tuesday as Brian Trainer, 27 — did not turn on his body camera until after the shooting, police said. In footage from Trainer’s camera that police made available Tuesday, Sterling can be seen lying on the ground astride his motorcycle and bleeding heavily.

In the background, a clearly alarmed bystander can be heard screaming “Oh my God!” At one point, the woman yells, “Michael Brown,” the name of an unarmed black teenager fatally shot by police in Ferguson, Missouri, whose death led to widespread protests and sparked a nationwide debate about use of force by police.

The video shows the officer scrambling to retrieve a first-aid kit for the man, who is bleeding heavily. The officers remove Sterling’s helmet and clothing and perform CPR. It’s not clear how soon after the shooting the CPR begins.

On Tuesday afternoon, a group of protesters marched to police headquarters from the nearby downtown intersection where Sterling was shot, about four blocks from the Washington Convention Center.

“The video pretty much shows what we already know it to be. We know that this is murder,” said Steven Douglass, 28, a youth minister and protest leader. “It was disheartening, it was disgusting, it was unaccountable. It was something that should never have taken place.”

The officer performing chest compressions on Sterling can be heard repeatedly urging Sterling to look at him and keep his eyes open. Sterling appears to be bleeding from a wound in the neck or upper chest.

Police have said they got reports about a motorcyclist driving erratically before officers stopped Sterling.

An attorney for Sterling’s family did not immediately return messages seeking comment Tuesday. Kevin Donahue, the district’s deputy mayor for public safety, said Sterling’s relatives were shown the video before it was released to the public.

The U.S. Attorney’s office and the police department are investigating the shooting. Trainer is a four-year veteran of the department.

In response to the officer’s failure to activate his body-worn camera, city 911 operators have been instructed to remind officers to turn their cameras on, and officers have been ordered to acknowledge the reminder. However, Donahue said Tuesday that the department has not had a widespread problem with officers not activating their cameras. In a monthlong span, cameras were activated 55,000 times, and officers failed to turn them on 10 times, he said.

The department is in the process of outfitting all patrol officers with cameras. Democratic Mayor Muriel Bowser has pledged to release body-camera footage when it’s in the public interest, and her administration has released video of previous shootings by officers.

The D.C. police union criticized the mayor’s decision to release the video and the officer’s name before the investigation is complete, calling it “reckless in the extreme” and harmful to a department that has shown a commitment to community policing.

“This decision places these officers in danger of misguided retaliation fueled by a false media narrative, and is a completely unacceptable action,” union chairman Matthew Mahl said in a statement. “These actions only show that Mayor Bowser places political expedience over the lives of the men and women who protect her and her constituents.”

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