WASHINGTON - A woman died after thick smoke filled a subway tunnel in the US capital Monday during the evening rush, officials said.
A spokeswoman for the Washington Metropolitan Area Transit Authority (WMATA) confirmed the incident, which caused rush-hour mayhem in the nation's second-busiest mass transit system after New York.
The Washington Post reported that several people were taken to hospital, including two with critical injuries.
Smoke filled the L'Enfant Plaza subway station in the centre of the city, forcing the evacuation of startled passengers and forcing afternoon rush-hour commuters to find other ways to get home.
Emergency workers escorted smoke inhalation victims to medical aid buses to receive oxygen.
WMATA said on its website that the "source of the smoke has not been determined" and that the National Transportation Safety Board has launched an investigation.
Photos posted on social media showed passengers crammed into smoke-filled trains covering their mouths and eyes, while the subway tunnel was engulfed with thick gray fog.
The station was temporarily closed. Service on two of the main city lines was partially suspended.
WMATA said emergency crews were deployed and that the station was being aired out.
"Metro Transit Police and fire department personnel are on scene at L'Enfant Plaza for smoke in the station," the metro system said on its website.
"Metro has activated tunnel fans to ventilate the area." The station is near several government buildings, office complexes and restaurants.
Washington's Metro system serves five million people in a 1,500-square-mile (3,885-square,kilometer) area, which includes transport to neighbouring Virginia and Maryland states.
The trains and buses provide about 2.3 million trips per year, according to WMATA.