The FBI agent who shot another passenger aboard a moving Metro train last month was first confronted by the man, warned him to back away and saw the man instead prepare to fight him, according to a 911 call by a witness to the encounter that was released Wednesday.

“The FBI agent said: ‘Move away. I’m an FBI agent. Back away,’ ” the 911 caller said. “The other gentleman didn’t, dropped his bag, approached him to fight him.”

The agent fired “two or three” rounds, the 911 caller said, as the train was pulling into the Medical Center station the morning of Dec. 15. The caller indicated that the agent then helped the injured man get off the train at the station.

The caller said the FBI agent was “attacked” by the other passenger but did not describe how and did not indicate whether there was any physical contact before the shooting.

The passenger was hospitalized with gunshot wounds but survived and has been released, Metro officials said. In the released recording, the 911 caller does not mention whether the man who approached the agent had a weapon. The FBI and Metro Transit Police have never indicated that a weapon was recovered from the scene.

Officials from the three investigating agencies — the FBI, the Metro police and the Montgomery County State’s Attorney’s Office — have declined to provide further details about what happened or release images from train-car cameras that captured the shooting. Days after the December shooting, authorities said it followed a “verbal exchange.” In the first 24 hours after the shooting, the FBI and transit police had called it only an “agent-involved shooting” and initially declined to answer questions about who specifically fired a weapon.

The state’s attorney for Montgomery County, John McCarthy, would not comment Wednesday on the 911 call or the case, citing his office’s ongoing investigation. Ian Jannetta, a transit police spokesman, declined to comment Wednesday other than saying the man who was shot had been released from the hospital and that Metro investigators have turned over their available evidence to the state’s attorney’s office.

Officials with the FBI public affairs office declined to comment.

The 911 recording, provided by Montgomery County’s 911 center after a public records request, offers only a limited snapshot — the account of one witness. It does not disclose what was said between the agent and the man who was shot.

But the call does offer the most detailed account to date of what transpired.

The witness placed his 911 call about 6:40 a.m. Dec. 15 and said there had been a shooting on a Metro train. In the recording provided to The Washington Post, some information, such as the caller’s phone number and name, is redacted. A few spots in the recording are difficult to hear.

“There’s a FBI agent on the train, on the Red Line pulling into Medical Center who . . . fired shots on the Red Line train,” the man says.

“And where are you?” the 911 operator asks.

“I’m on the train that it occurred in,” the man says, adding, “There’s a law enforcement officer who shot a person who attacked him.”

The man continues speaking as the train stops at the Medical Center station. The bells of opening train doors can be heard in the background. The caller indicates he is watching the FBI agent help the man he shot get to the platform.

“He’s trying to get the person off the car,” the man says. “The law enforcement officer and the person shot are now off the train, both of them.”

The caller stays on the train as it pulls away, indicating he wanted to avoid following them onto the platform.

“How many shots were fired?” the operator asks.

“Two or three,” the caller says.

While continuing south on the train toward the District, the caller describes more of what he saw.

“A gentleman came on the train at the Grosvenor train station and confronted the gentleman who was sitting on the train, got in his face,” the caller says. “He didn’t have a mask.”

He says the FBI agent had been seated.

“He was sitting in the second car by himself, and the gentleman approached him without his mask on, just kind of hounding him,” the caller says. “Somehow, I was in the back of the car. They confronted each other.”

Moments later, the 911 operator asks whether anyone was currently in danger.

“I don’t think so,” he says. “I’m no longer with them.”