WASHINGTON — Smoke from an unidentified source filled a busy downtown subway station in the nation’s capital Monday, forcing the evacuation of hundreds just before afternoon rush hour, authorities said. One person died and dozens were injured.

Hundreds of passengers were evacuated from the L’Enfant Plaza station, one of the subway system’s busiest, about 3:30 p.m., said District of Columbia Fire and EMS spokesman Timothy Wilson.

Fire and EMS spokeswoman Caroline Laurin confirmed that a person had died, but declined to say more. She said the National Transportation Safety Board, whose office is located at L’Enfant Plaza, was investigating and would provide additional information. The NTSB tweeted that it had begun an investigation and was on the scene.

“It started to get scary pretty quick,” passenger Jonathan Rogers was quoted by The Washington Post as saying. “People started praying. Smoke was coming in pretty steadily.”

Passenger Saleh Damiger was quoted by the newspaper as saying that people were choking and yelling aboard the train. “It was a lot of smoke,” she said. “We couldn’t see each other. ... We felt like we were almost going to die.”

Eighteen people from the station were taken to Medstar Washington Hospital Center, most of them for smoke inhalation, according to spokeswoman So Young Pak. She said 11 were treated and released. Of the seven still in the hospital Monday night, one was in critical condition and one was in serious condition, she said.

George Washington University Hospital spokesman Matt Brock said in an email that 34 patients suffering from smoke inhalation had been brought there. He said their conditions varied.

The Post and ABC 7 television reported that 40 people were taken to Howard University Hospital.

The source of the smoke was not immediately known.