WJLA-TV, Associated Press
In this still image taken from video, police and fire officials stand outside the entrance to the Potomac Avenue Metro Station, Thursday night, March 12, 2015 in Washington. Transit police in Washington, D.C. fatally shot a man after responding to a call about an "unauthorized person" on the tracks in a subway tunnel.

WASHINGTON — 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.

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."

Officials declined to say Friday whether the man was armed or how he may have threatened the police officer. Police have not identified the man.

No officers were injured in the shooting, said Metro Transit Police spokesman Mike Tolbert.

A District of Columbia police homicide unit was leading the investigation into the shooting Friday, though police released few details. Investigators said the man was apparently partially clothed. The FBI's Washington field office said Friday that it was not involved in the investigation.

The officer involved was placed on routine paid administrative leave during 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 it's rare for an incident 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. Police said Chester J. Crestwell Jr., 44, fired at the detective several times and the detective returned fire. Crestwell died at the scene, and a handgun was found there. The detective wasn't hurt.

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.

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