Federal prosecutors said Thursday that they will not file charges against the D.C. police officer who fatally shot 18-year-old Deon Kay in September during an attempted arrest.

Prosecutors said Kay was holding a gun and had raised his arm about the same moment that Officer Alexander Alvarez fired. Kay tossed the weapon.

Prosecutors in the U.S. attorney’s office said they could not determine beyond a reasonable doubt that Alvarez committed “willful violations” of federal criminal civil rights statutes.

Police said they approached a group of men, including Kay, on Sept. 2 after video live-streamed over social media showed them with guns inside a parked Dodge Caliber in Southeast Washington. As police arrived, the men fled.

Prosecutors said a review of Alvarez’s body-camera footage showed that when the officer encountered Kay, the youth had a gun in his right hand and raised his arm at “approximately the same instant that the officer fired.” Kay was struck once in the chest. Kay then tossed the firearm, which was found about 98 feet away. Prosecutors said they were unable to determine whether Kay threw the gun deliberately or reflexively on being shot.

“The U.S. Attorney’s Office is unable to disprove a claim of self-defense or defense of others by the officer involved, who fired a single shot at Mr. Kay within one second of Mr. Kay holding a gun in his hand and raising his arm,” the office said in a statement.

Kay’s death, which sparked demonstrations, also gave new urgency to an effort to address policing strategies that critics say can lead to police use-of-force excesses, particularly against Black men. The District had already created a Police Reform Commission tasked with reimagining policing in D.C. in light of protests for racial justice.

Prosecutors in the public corruption and civil rights section of the U.S. attorney’s office worked with the police department’s internal affairs division in examining the case. In addition to Alvarez’s body-worn-camera footage, the prosecutors reviewed statements from officers and civilians, radio transmissions and the autopsy report.

Kay’s mother, Natasha Kay, declined to comment on the finding, saying she was acting on the advice of her attorney.

Because of the concern in the community, completing the investigation was a priority for prosecutors. They reached their conclusion within two months, much faster than in previous police shootings.

Acting U.S. attorney Michael Sherwin said more manpower than usual was dedicated to the investigation in part because of changes implemented over the summer after 32 Black prosecutors in the office voiced concerns about aspects of its workings, including how cases involving alleged excessive use of force by officers were handled.

As a result, the office reorganized its civil rights unit to allow its eight to 10 attorneys, along with investigators and para­legals, to jointly investigate such cases. Previously, two or three attorneys were assigned to each case.

In a statement, Sherwin said the office had “prioritized our investigation of potential civil rights violations, in general, and excessive force matters, in particular.”

“We believe that, while we must and will thoroughly and appropriately investigate potential instances of excessive force, it’s in the community’s interest that we resolve such investigations expeditiously,” he said.

City records have shown that prosecutors in the U.S. attorney’s office have never filed criminal charges against a D.C. police officer involved in a fatal on-duty shooting.

D.C. police said the department will begin an administrative investigation to determine whether Alvarez followed police rules.

Prosecutors in the civil rights section also are likely to review the Oct. 23 crash in the Brightwood neighborhood of Northwest Washington that resulted in the death of Karon Hylton.

Police said Hylton, 20, had been seen riding a moped on a sidewalk and without a helmet. He was being closely followed by a police vehicle in an alley when he emerged onto a street and was fatally struck by a van. Investigators were trying to determine whether officers in the cruiser were chasing the moped. D.C. police regulations prohibit officers from engaging in pursuits for traffic violations.