A woman has died and dozens of people have been injured after thick smoke filled a subway tunnel in the US capital during the evening rush.
A spokeswoman for the Washington Metropolitan Area Transit Authority confirmed the incident on Monday, which caused mayhem in the second-busiest US mass transit system after New York.
Washington emergency medical services said 84 people were taken to hospital with "various injuries" and that one firefighter was also injured. About 200 people were being evaluated, it added.
The Washington Post reported two people were suffering from critical injuries.
WMATA said emergency crews were deployed and that the station was being aired out.
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"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."
Smoke filled the L'Enfant Plaza subway station in the centre of the city during rush hour, prompting the evacuation of startled passengers and forcing post-work commuters to find other ways to get home.
Emergency workers escorted smoke inhalation victims to medical aid buses to receive oxygen.
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 grey fog.
WMATA said on its website that the "source of the smoke has not been determined" and that the National Transportation Safety Board had launched an investigation.
NTSB confirmed it was leading the probe, and said on Twitter later on Monday "investigators are on the scene".
The station was temporarily closed on Monday before reopening for partial service, but transport authorities said trains could be delayed on Tuesday.
