A D.C. police officer who fatally shot a young man Wednesday afternoon in Southeast Washington fired once after seeing the 18-year-old turn while holding a gun with his right hand, according to body-camera video made public Thursday.

Deon Kay, who lived with his mother a half-mile from where he was shot in Congress Heights, was struck in the chest and the bullet exited his back, authorities said. He died at a hospital.

D.C. Police Chief Peter Newsham said officers in the 7th District saw a live-streamed social media video of Kay and others inside a vehicle, holding weapons. Newsham said the officers recognized Kay and another man from previous interactions.

They tracked the vehicle, a Dodge Caliber, just before 4 p.m. to a parking lot in the River Hill apartment complex in the 200 block of Orange Street SE, police said.

One of the occupants exited the vehicle and was chased by police before escaping. Police said the officers then saw Kay, who then got out of the vehicle. He is seen on the video holding a gun and turning to face the officer who was running toward him.

“Don’t move! Don’t move!” the officer, identified as Alexander Alvarez, is heard shouting on the video. Alvarez fired one shot.

Newsham said Kay threw the weapon as or just after he was struck. It was found nearly 100 feet away in a grassy lot at the apartment complex.

District officials said Alvarez, who is in his mid-20s and joined the force in 2018, was assigned to the crime suppression team in the 7th District.

Officials said a complaint was previously filed against him with an independent review board and was dismissed. Officials did not elaborate. He has been placed on routine leave pending the outcome of the investigation.

Efforts to reach Alvarez on Thursday were unsuccessful. A representative of the police union did not respond to an inquiry.

District officials moved quickly to try to tamp down demonstrations like those that been a fixture in the city and across the country since George Floyd was killed in the custody of Minneapolis police in May. As has been true elsewhere, some of the demonstrations in the District have turned violent.

Demonstrators converged at the 7th District police station Wednesday night and outside the home of Mayor Muriel E. Bowser (D) on Thursday morning.

At a news conference, Newsham said there “is a lot of misinformation” about the shooting.

Bowser said people should “not let any anger spill over into violence in our city.”

The investigation is just beginning, authorities said.

“The officer was trying to take guns off the street,” Bowser said. “He encountered somebody with a gun.”

Kay’s family and others have questioned the police account of the shooting. Demonstrators on Thursday demanded Bowser fire Newsham. A police spokesman said family members watched the video of the shooting before it was made public. Bowser said the city has offered relatives counseling.

Attempts to reach Kay’s relatives by phone, at their home and through one of his mentors who knows the family, were unsuccessful Thursday.

Newsham, at the news conference, said Kay was a “validated gang member” who had “multiple touches with the criminal justice system.” The chief’s assertions could not be verified, and he said he could not elaborate on specific cases. He said when officers saw the live-streamed video, they knew Kay by name.

“This is a tragedy to have any young person killed in the District of Columbia,” Newsham said. “I’m pretty sure Deon Kay fell through multiple safety nets before yesterday afternoon.”

Police arrested two other people allegedly from the vehicle and said they found an additional firearm on one of the men charged. Police described that weapon as a “ghost gun,” made from kits and lacking serial numbers.

Marcyelle Smith, 19, of Southeast was charged with carrying a pistol without a license. A D.C. Superior Court judge on Thursday ordered him released and set a court day for January.

The judge said Smith had no prior convictions. Through his attorney, Smith described himself as rapper who has been in several music videos that are on the Internet and that showcase firearms.

The other man arrested was charged with not carrying a driver’s license.

D.C. Council member Trayon White Sr. (D-Ward 8) noted the tensions between police and residents of his ward, which includes neighborhoods dealing with economic challenges and high crime rates.

“I want to get the truth out about what happened,” White said. “We need to figure out what the facts are. We are concerned.”

On Wednesday night, Kay’s mother, Natasha Kay, said police had told her very little about the shooting.

“I need my son back,” she said. “I want my son back.”

Omar Jackson, a youth mentor who had worked with Deon Kay for the past two years, said the young man “was trying to navigate through this chaotic situation out here.” He said Kay had just gotten into a high school equivalency program. Kay had previously attended the Ballou STAY Opportunity Academy, according to city officials.

Jackson said Kay liked to spend time with his girlfriend and play sports. Kay also took care of his nieces and nephews, Jackson said.

“He was trying to get himself together and get out of this situation,” Jackson said. “I feel bad. My job is to keep him out of situations like this.”

The video from Alvarez’s body camera moves fast, and it is difficult to see the weapon police said Kay brandished. The video D.C. police made public slows the footage down and stops at the point where the gun is visible.

An arrest affidavit for Smith provides a detailed version of the police account. It says the officer saw “Kay raise his right hand from the right side of his waist. As Kay’s right hand rises, an object consistent in appearance with a handgun is visible in his hand.” The affidavit says that Kay’s arm “continues to rise” and that an “object comes out of his hand.” The document does not say at what point Alvarez fired.

A police report says Kay “posed an imminent deadly threat . . . by brandishing a firearm in the direction” of the officer.

After the shooting, the video shows an officer — who a police spokesman confirmed was Alvarez — searching for the firearm.

“He threw it,” the officer identified as Alvarez is heard saying on the video as he walks through a grassy lot near a small playground. “I got to find it.”

Alvarez found the firearm a few moments later, police said, about 98 feet from the shooting scene. Newsham said the gun traveled that far in part because it was downhill.

Keith L. Alexander, Katie Mettler, Emily Davies and Steve Thompson contributed to this report.