The station was temporarily closed after filling with smoke, while a disabled train was evacuated. D.C. Fire and EMS tweeted that one person was critically injured and six others were hurt in the incident after a train filled with smoke and its passengers were evacuated.
The National Transportation and Safety Board was investigating the incident and Metro Transit Police and fire department personnel were at the station examining the problem, WMATA said, but had not yet found the source of the problem as of Monday afternoon.
WMATA spokeswoman Morgan Dye told CNN Monday evening that while everyone in the station had been evacuated, they still were searching for the source of the smoke.
The smoke was thick enough to prompt WMATA to suspend service on several lines, including parts of the Green and Yellow line, which both stop there. Blue, Orange and Silver lines were bypassing the stop.
Shots of the scene showed numerous emergency vehicles idling outside the complex of office buildings that make up L'Enfant Plaza.
The buildings largely house private businesses, but the U.S. Postal Service is also headquartered at L'Enfant Plaza.
Alec Dubois, a freshman at Gonzaga College High School, told CNN affiliate WJLA he was in a tunnel on a train near the station when smoke started filling the air.
"People were freaking out, going everywhere. The conductor was telling us to stay calm. He said he was trying to get our train back to the station. We had no cell service. I was trying to call my parents. And then there was black stuff everywhere, it was all over my pants, too. It was under our noses. We have no idea what it was."
"We decided to go out one of the side doors and then we started walking on the rails," he said. "Once we got out of the station, we just sprinted for our lives...because we had no idea what was happening."
He described the smoke as "translucent."
Lesley Lopez, a metro rider who was there when the smoke broke out, said she "wasn't sure what was happening, and then I heard a voice say, 'Get out of the train station.'"
The longtime metro rider said, however, the evacuation was "poorly managed," and that she was "disturbed" by the fact she was given no evacuation instructions or indication of what had happened.
"It's disheartening because there did not appear to be an emergency plan," she said.
D.C. Mayor Muriel Bowser issued a statement following the incident thanking city first responders and offering her condolences to the family of the dead passenger.