An earlier version of this story incorrectly stated that the Justice Department entered the criminal case against two U.S. Park Police officers to assist the defense. It entered the case as an “interested party” and has not yet filed its position. The story has been corrected.

The Justice Department, in a letter signed by Attorney General Merrick Garland, has reversed a decision made during the Trump administration and will allow FBI agents to cooperate in the manslaughter prosecution of two U.S. Park Police officers after the fatal shooting of Bijan Ghaisar.

The change of heart could provide a big boost to Fairfax County prosecutors, and lawyers for the Virginia attorney general who’ve joined the case, who will now have access to the agents who investigated the November 2017 shooting of Ghaisar by Officers Lucas Vinyard and Alejandro Amaya. After a short pursuit on the George Washington Memorial Parkway, during which Ghaisar stopped and then pulled away twice, Ghaisar drove into a neighborhood in the Fort Hunt area and stopped a third time. When he started to roll away again, a video shows, Vinyard and Amaya fired 10 times, killing the unarmed Ghaisar.

The FBI handled the case because the Park Police are federal officers. After a two-year investigation, the Justice Department’s civil rights division decided in November 2019 not to charge Vinyard and Amaya. Then in February 2020, Assistant Attorney General Eric S. Dreiband informed Fairfax prosecutors that the FBI would not be allowed to cooperate with a local investigation because the department might be defending the officers, and that would be a conflict of interest.

Fairfax prosecutors pursued the case anyway, without the agents who performed the investigation or all of their case files. And last October a special grand jury indicted Vinyard, 39, and Amaya, 41, on charges of involuntary manslaughter and reckless use of a firearm. The officers’ lawyers then moved the case to federal court in Alexandria, where federal law allows federal agents to be tried. Virginia Attorney General Mark Herring assigned some of his attorneys to join Fairfax in the case, at the request of Fairfax Commonwealth’s Attorney Steve Descano.

After the Trump administration gave way to the Biden administration, Herring and Descano wrote a letter last month to the newly installed attorney general, asking the Justice Department to reconsider both the decision not to charge the officers and the decision not to cooperate with Fairfax. They said that in-car camera footage captured by a Fairfax police cruiser showed “yet another young person of color being killed by law enforcement,” and that “the most significant impediment” to obtaining justice in the case was “the Trump Administration.”

Garland’s response, dated June 1 and obtained Monday by The Washington Post, said he had authorized Justice personnel “to contact the Commonwealth’s Attorney immediately to discuss your requests for access to information about the federal investigation and any evidence in the possession of the FBI. We will share with the Commonwealth all appropriate information and evidence,” consistent with procedural rules on federal cooperation with local investigations.

Garland’s letter did not discuss whether the Justice Department was reconsidering its decision not to charge the officers, or the December decision to enter the manslaughter case as an interested party. The Justice Department is defending the Park Police in a wrongful-death suit filed by Ghaisar’s family, in which the officers were dismissed as defendants. The officers remain on paid leave.

A Justice Department spokeswoman declined to comment on why the department had changed its mind.

“We welcome the Justice Department’s decision to further cooperate in the case against the U.S. Park Police officers who took Bijan Ghaisar’s life,” Herring and Descano said in a joint statement. “We remain fully committed to pursuing justice for the family and community in this case, and are encouraged that the Justice Department is cooperating with this pursuit.”

Lawyers for the officers declined to comment on Garland’s letter.

“This is a preliminary step in the Ghaisar family finally getting justice,” said Thomas G. Connolly, a lawyer for the Ghaisars. “But that’s all it is, a first step.” Connolly, a former federal prosecutor, also sent a letter to the Justice Department seeking reconsideration of the case, saying that “firing repeatedly at an unarmed man through the window of his vehicle after responding to a minor traffic accident [is] certainly sufficient to support a prosecution.”

The next big step in the case is scheduled for August. That’s when a hearing is set to determine whether a federal judge should dismiss the Fairfax charges because the federal officers can’t be charged in state court if their actions were “necessary and proper.” Fairfax prosecutors said they probably would take about a week to put on evidence that the shooting was neither necessary or proper. The “supremacy clause” of the U.S. Constitution holds that states must defer to federal law, and the officers have argued that they have already been cleared by the Justice Department, and acted properly because they feared Ghaisar would run over Amaya.

Now the FBI agents who originally investigated the case could testify in that hearing, rather than the Fairfax officers and detectives who picked up the case after the FBI was barred from participating.

Lawyers for both officers filed their motions to dismiss the case on Tuesday evening, and asked U.S. District Judge Claude M. Hilton to do so without holding a hearing. Both argued that Ghaisar acted erratically, had committed crimes by repeatedly fleeing them and drove his Jeep Grand Cherokee toward Amaya. “Ghaisar placed Officers Amaya and Vinyard in a life-or-death situation,” attorneys Travis Tull and Jonathan Fahey wrote for Amaya, saying the officer “believed he was in danger of dying if he did not take the action he took when he took it.”

When the Justice Department’s civil rights division decided not to charge Vinyard and Amaya with federal criminal civil rights charges, the announcement noted that the decision did not preclude anyone else from investigating the case.

So then-Fairfax prosecutors Raymond Morrogh and Casey Lingan in late 2019 quickly obtained many of the FBI files, worked with FBI agents to build a case, and were prepared to seek murder indictments against Vinyard and Amaya in December 2019. But the Justice Department would not grant permission for the FBI agents to testify.

After Descano took over as Fairfax prosecutor in January 2020, he was soon greeted by the letter from Dreiband, responding to the request for FBI cooperation made by his predecessors. Dreiband said the Interior Department, on behalf of the Park Police, had asked the Justice Department to defend both the wrongful-death civil suit filed by the Ghaisars and a possible criminal case. Dreiband said the Justice Department was considering both requests, and ultimately it did enter the civil case to defend Park Police.

“The Department is unable to authorize Department employees,” Dreiband wrote, “including FBI agents, to appear before a state grand jury in Fairfax County.” Dreiband said prosecutors could use Fairfax police to review the FBI case files, “conduct their own investigation, and testify.”

That is the course Descano took, enlisting veteran Fairfax homicide detective Chris Flanagan to review the case and present it to a special grand jury. Two Fairfax patrol officers who followed the pursuit and were present at the shooting also are available to testify.

Also last year, Descano sent a letter to Dreiband requesting hundreds of documents missing from the FBI case files that had been provided to Fairfax and other redacted information, and there has been no indication that the FBI cooperated with that, either. Garland’s letter seems to indicate that the FBI will now provide those documents.