WASHINGTON (AP) — A transit police officer fatally shot a man officials said was found trespassing in a Washington subway tunnel, and police were investigating Friday but did not specify what kind of threat the man posed.

The Metrorail Operations Control Center reported an "unauthorized person" on the tracks Thursday night outside of the Potomac Avenue station, about two stops from the U.S. Capitol, just before 9 p.m. A train operator first saw the man in the tunnel, and that track was taken out of service, Metro officials said.

Metro Transit Police officers responded. One officer encountered the man about 400 feet from the station along the westbound tracks between Potomac Avenue and the RFK Stadium stop, Metro spokesman Dan Stessel said.

"Shots were fired by the officer using a department-issued service weapon" and the man was pronounced dead at the scene, Stessel said. "It is not yet known how the man entered the tunnel, what he was doing there, why, etc."

The Metropolitan Police Department identified the man as 35-year-old Bobby Gross.

Officials declined to say Friday whether the man was armed or how he may have threatened the police officer.

No officers were injured in the shooting, said Metro Transit Police spokesman Mike Tolbert. It was not clear whether the officer used any nonlethal force against the man prior to shooting. All Metro police officers carry pepper spray and an impact weapon, such as a baton or nightstick, in addition to a firearm, Stessel said.

The officer involved was a veteran officer, a woman with more than 10 years of service on the force, Metro said. She was placed on routine paid administrative leave during the investigation.

A District of Columbia police homicide unit was leading the investigation into the shooting, though police released few details. Investigators said the man was apparently only partially clothed.

Police spokesman Lt. Sean Conboy said investigators would release little information because the shooting was being treated as a criminal investigation.

"It's a crime until it's not a crime," Conboy said.

Metro said it was reviewing available surveillance video and will turn over footage to the U.S. Attorney's office and D.C. police for their review of the use of deadly force.

The FBI's Washington field office said it was not involved in the investigation.

The Metro station was closed after the shooting, and trains that normally stop there bypassed the station. But the station reopened at its usual time Friday morning.

Metro Transit Police officers have been involved in fatal shootings before, though this was only the second use of deadly force in the past decade, Stessel said. It is also unusual for a shooting to unfold inside a subway tunnel.

In 2012, a Metro Transit Police detective fatally shot a man in a suburban Lanham, Maryland, neighborhood while investigating an earlier abduction attempt at a Washington bus stop.

In 2003, a Washington man was fatally shot after he fled on foot from a stolen car and struggled with an officer. Prior to that shooting, the last fatal shooting by a Metro transit officer was in 1999.

The Metro Transit Police Department has 490 officers, according to Metro's website. Metro said the department is unique in that it has authority in three jurisdictions: the District of Columbia and parts of Maryland and Virginia. Officers provide law enforcement on the system's subway and bus networks.

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Associated Press writers Sarah Brumfield and Jessica Gresko contributed to this report.