WASHINGTON (AP) — A spokeswoman for the metro system in Washington, D.C., says one person has died after smoke forced the evacuation of one of the U.S. capital’s busiest downtown stations.

Spokeswoman Caroline Laurin declined to give more information Monday evening, saying the National Transportation Safety Board was investigating.

Hundreds were evacuated from the L’Enfant Plaza station about 3:30 p.m. Monday. At least six were taken to the hospital.

District of Columbia Fire and EMS spokesman Timothy Wilson said earlier in the day that emergency medical workers evaluated dozens of passengers after smoke filled the station.

The source of the smoke was not immediately known.