Smoke filled a busy Washington D.C. subway station, killing one and injuring 84 others, prompting an investigation by the National Transportation Safety Board.

A woman died and many were injured aboard a Yellow Line train that was headed to Virginia and had just departed from L’Enfant Plaza station at around 3:30 p.m., close to rush hour. Suddenly, it stopped about 800 yards past the platform and smoke started to fill the cars, according to a Fox News report.

NTSB officials said the smoke was from electrical arcing about 1,100 feet in front of the train, which is when water his the electrified third rail and creates plumes of smoke. Authorities later found that close to an inch of water was on the ground next to the third rail, but they were not sure why the train had suddenly stopped.

Authorities don’t think there was a fire, and power was shut off to the third rail, focing passengers to evacuate themselves. It was an hour before firefighters could get to them, according to some passengers, who were choking as smoke began to fill the cars, prompting some to lose consciousness.

Firefighters weren’t able to immediately enter the car until the third rail had been shut off, according to D.C. Interim Fire Chief Eugene Jones, and disputed passengers’ accounts of it taking an hour for firefighters to get there.

NTSB said their investigation would continue Tuesday morning, and rail service into the city would probably be impacted by that investigation.

Two people remained hospitalized in critical condition on Monday night. Over 200 people were examined in all, including a firefighter.

George Washington University Hospital reported that a total of 34 patients had been brought there, all suffering from smoke inhalation injuries.