Trainer, 28, was climbing out of the car to make a traffic stop when Sterling "revved his motorcycle and then accelerated" toward the cruiser, the U.S. attorney's office said in a statement. The officer felt the motorcycle hit the door and "reacted by immediately firing two rounds at Mr. Sterling through his front passenger window." Sterling was struck in his neck and side.
That night, Sterling had reached speeds of more than 100 mph and run red lights, including one instance in which he sped through a stoplight after looking at the officers, according to the statement. An autopsy determined that Sterling's blood alcohol level was twice the legal limit, and he tested positive for marijuana.
"After a careful, thorough, and independent review of the evidence, federal prosecutors have found insufficient evidence to prove beyond a reasonable doubt that the officer willfully used unreasonable force and/or was not acting in self-defense when he discharged his service weapon at Mr. Sterling," according to the statement from the office of U.S. Attorney for the District Channing D. Phillips.
No D.C. police officer has ever been charged criminally for a fatal on-duty shooting, according to a city-sponsored 2016 report on the District's police shootings titled, "The Durability of Police Reform: The Metropolitan Police Department and Use of Force." The report was commissioned by D.C. Auditor Kathleen Patterson.
Although Trainer will not face criminal charges, D.C. Mayor Muriel E. Bowser (D) said the police department has asked him to resign. "I do not believe there can be real accountability if the officer remains on the force," Bowser said in a statement.
Trainer, who has been on the force for four years and is on administrative leave, faces an internal investigation into whether his conduct before and at the time of the shooting conformed with department policy. He could not be reached for comment.
Wednesday's announcement came after prosecutors met for nearly two hours with Sterling's father, Isaac, and the family's attorney, Jason Downs, to explain their findings.
"My son was loved by his family, his community, and the community at large. You all have not heard yet the enormity of the loss of my son," Isaac Sterling said outside the U.S. attorney's office. "He was a very well-loved person."
Downs said he was "outraged" to learn from prosecutors that they presented the case to a grand jury but did not allow it to take a vote on whether to charge the officer.
"That is disappointing and frustrating. It frustrates the purpose of our system," Downs said. "After sitting through so much evidence, grand jurors should have been given the right to vote this up or down. The prosecutor's office took that decision away from them."
William Miller, a spokesman for the U.S. attorney's office, declined to speak about Downs's comments. But prosecutors have said it is not uncommon to use a grand jury to investigate a case and that the government does not always seek an indictment.
The decision not to charge prompted immediate response from some local political leaders. Charles Allen (D-Ward 6), who chairs the council's Committee on the Judiciary and Public Safety, said that he is "frustrated" that the community has waited months for action from prosecutors only to reach a decision that he expects will cause a rift between residents and law enforcement.
"I think it has done a lot of damage, and it takes a lot to heal from that," Allen said of the prosecutors' decision and the shooting.
Rep. Eleanor Holmes Norton (D) criticized the decision not to charge the officer.
"We have seen far too many police shootings of unarmed African Americans in our country, and the public is frustrated by the difficulty in getting prosecutions of officers," she said in a statement.
Questions arose over the circumstances of the shooting, partly because Trainer did not activate his body-worn camera until one to three minutes after the shooting. In an unusual move, Bowser broke with police policy after the incident and ordered Trainer to be publicly identified to demonstrate transparency amid concern from the public.
Although Sterling's case never gained the national attention of some other police shootings of unarmed suspects, marches and vigils were held in the weeks and months afterward. On Wednesday night, several dozen people gathered near the scene of the shooting, where they prayed, sang and called on the city to fire Trainer.
The case may still be heard in civil court. Sterling's family filed a $50 million lawsuit against the District and its police department.
The family alleges that Trainer and his partner may have violated D.C. police regulations. They question whether the officers improperly chased Sterling, whether the police cruiser was improperly used as a barricade and whether Trainer may have improperly fired at a moving vehicle or improperly fired from within a cruiser.
His family believes that on the night of his death, Sterling, a motorcycle enthusiast who worked as an HVAC technician, had been at a party and was headed to the home he shared with his parents.
The incident began about 4:20 a.m. on Sept. 11, when officers got a call about a motorcycle being driven erratically in the Adams Morgan area. Later, police saw the motorcycle near Third and M streets in Northwest, near the Third Street Tunnel, and pulled into an intersection ahead of the biker.
Prosecutors determined Trainer had unholstered his gun, keeping it pointed down and toward his body, as he began to get out of the car. Sterling drove toward the cruiser, and "the impact caused by the advancing motorcycle caused a dent in the cruiser's open door and a bruise to the officer's knee," prosecutors said in the statement.
Two witnesses interviewed by The Washington Post said that the crash did not appear deliberate and they thought that Sterling was trying to move around the police car.
One witness, Howard Dorsey Jr., was waiting at a stoplight when he saw Sterling straddling his motorcycle and then walking the cycle into the crosswalk ahead. Seconds later, Dorsey said, a police car pulled into the intersection, blocking the left lane.
Dorsey said the motorcyclist accelerated to between 5 and 10 mph. The right lane was open, he said, but the biker steered left.
Just then, a police officer opened the front passenger door a few inches, Dorsey said. He said the motorcycle's front tire hit the door with enough force to dent it, but no more.
"That's when he let off the shots," Dorsey said. "The cops said no words, nothing. No 'freeze.' "
The shooting prompted D.C. police to update their policy on body cameras, and officers are now required to confirm with dispatchers that their cameras are on when they respond to calls.
Ellie Silverman and Wesley Lowery contributed to this report.