Protesters blocked an intersection in downtown D.C. on Oct. 3 to demand the release of more information about the police shooting of Terrence Sterling, an unarmed black man. (Video: WUSA9 / Photo: Ricky Carioti/The Washington Post)

What next in the investigation of unarmed black motorcyclist Terrence Sterling, who was shot and killed on Sept. 11 by D.C. Police Officer Brian Trainer?

Speaking to reporters last week, Jason Downs, an attorney for Sterling’s family, said the 31-year-old was killed “unlawfully and unjustifiably.”

“It appears that Officer Trainer fired his weapon from the safety of his police vehicle when Mr. Sterling did not pose any threat to him whatsoever,” Downs said. The family, he said, wants more information about the circumstances surrounding the shooting, which took place near Third and M Streets NW.

Downs and Sterling’s family aren’t alone.

Hundreds of protesters have taken to city streets, holding vigils and blocking intersections to draw attention to the shooting and demand answers. Other District residents behind closed doors want answers, too.

The frustration is understandable. Three weeks have passed since news broke about the fatal shooting.

Here’s the problem: In all likelihood, weeks — if not months — will elapse before the Sterling family and the public learn anything new. The tragic event is in the hands of D.C. officials and the U.S. attorney’s office.

And federal prosecutors are reluctant to speak about the investigation in its early stage. There’s a history of tight-lipped probes, too.

There have been three police-involved fatalities in the District in the past 12 months. Each was treated by prosecutors the same way, with months elapsing before conclusions were announced.

For example, on Sept. 30, 2016, the U.S. attorney’s office announced that it had completed the review of a fatal shooting by an off-duty Baltimore County, Md., cop that occurred on Nov. 14, 2015, at Union Station. Prosecutors said they found insufficient evidence to pursue charges against the officer.

Next, there’s the investigation into a fatal shooting by a D.C. police officer at an intersection in Northeast D.C. on Nov. 19, 2015. It wasn’t completed until June 24, 2016, when prosecutors announced that they also found insufficient evidence to press charges in that case.

Results of the federal probe into the Sept. 29, 2015, fatal encounter between a hospital patient and two special police officers outside the hospital were announced seven months later, on May 17, 2016. The two officers were indicted by a grand jury on a charge of involuntary manslaughter.

In all police-involved fatalities, the U.S. attorney’s office investigates to see whether there is evidence to show that officers violated either federal or D.C. law. In two of the preceding cases, federal prosecutors, as explained in press releases, were unable to prove beyond a reasonable doubt that the force used was excessive and “that the officer acted with the deliberate and specific intent to do something the law forbids.” “Prosecutors must,” as stated in the press releases, “be able to prove that the officers involved willfully used more force than was reasonably necessary. Proving ‘willfulness’ is a heavy burden.”

So in the Sterling case, expect the U.S. attorney’s office, assisted by the D.C. police department’s Internal Affairs Division, to conduct its probe in the same way it approached the three other cop-involved shootings — interviewing witnesses (both cops and civilians); combing through physical evidence, videotapes and recorded communications; and examining DNA and autopsy reports. All are time-consuming actions.

But months?

Folks are demanding answers now.

The U.S. attorney must give this probe top priority and get results to the family and public. That’s all the more important because the police already have made public their version of what happened to Sterling.

They said that a motorcyclist was driving “recklessly” and that “when the officer was exiting the passenger side of his marked police cruiser to stop the driver, the motorcyclist intentionally drove into the passenger door and the officer fired his weapon.” Thus, cops placed the blame on Sterling.

Now here’s the problem and why it’s so important to expedite the investigation.

Some witnesses dispute the police department’s account, according to FOX5 news and NBC4. They say Sterling didn’t try to intentionally run into the cruiser and that the cop was inside the cruiser when he pulled the trigger.

Since releasing footage from the body-worn camera on Trainer, which was not activated until after the shooting, and repeating instructions that officers should turn the cameras on in civilian encounters, Mayor Muriel E. Bowser and her staff have hit the “mute” button.

In part, that’s understandable. The investigation must be independent, thorough and impartial.

But the police know stuff. The carefully worded statement released on the shooting was based, in part, on what was said by the officer who fired his weapon and the officer who drove the cruiser. Presumably, Interim Police Chief Peter Newsham, a 27-year police veteran who wants the top job, has briefed his boss, Deputy Mayor for Public Safety Kevin Donahue, who in turn must have briefed Bowser about what happened that day.

Don’t blame the public for thinking that city officials may know something, too.

Preserving the sanctity of the investigation is critical. So, too, however, is the need for accountability between the police and the community, and open and transparent government.

And so to the question “What next?”: Whatever it is, it had better come soon.