A Metro transit employee was shot and killed Wednesday on a Washington, D.C., train station platform while trying to stop a gunman who had already shot and wounded others.

The incident began as an altercation onboard a Metrobus near 14th Street and Potomac shortly after 9 a.m., Executive Assistant Chief of Police Ashan M. Benedict said at a press conference.

A person with a gun was engaging passengers on the bus and followed one of them off before shooting them in the leg.

Police say the gunman then went down the Potomac Avenue Metro station escalator to the station platform and approached someone who was trying to buy a Metro card. Benedict said the gunman shot that person and attempted to bring them over the faregate turnstile.

The gunman then allegedly went further down the escalator, where he confronted a woman. That is where a Metro transit employee who was nearby attempted to intervene, but was shot and killed by the gunman.

"A second Metro transit employee then approached the shooter who entered and exited the stopped train before being taken into custody by officers," FOX 5 DC reports. 

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Three people were injured in the attack and taken to area hospitals, Benedict said. Two victims suffered leg injuries, one had an injury to their hand. The transit employee who was killed has not yet been identified. 

The shooter is in custody. Anyone who witnessed what happened is asked to contact police.