One woman is dead, dozens of people were hospitalized and three remain in critical condition after the upper level of the L'Enfant Plaza Metro station in southeast D.C. filled with smoke Monday afternoon.

A Virginia-bound Yellow Line train was in the tunnel just south of the station around 3:20 p.m. when it stopped for unknown reasons.

Dozens of people were trapped inside the train's cars as the tunnel filled with thick, black smoke.

"People could barely breathe," passenger Denzel Hatch said. "They had to evacuate us through the tunnel and walk back through the front. No electricity, no visibility, nothing. Couldn't see anything at first."

"You're hearing other people crying, you're seeing people passing out... it was a horrific thing to witness," passenger Adjoa Adofo said.

A young black man had a seizure. Thank God the firefighters had made it on to the train by then. We were trying to escape through the back A video posted by Adjoa, Inc © (@adjyb) on Jan 12, 2015 at 2:13pm PST

Firefighters gained access to the tunnel at one of the evacuation points at Ninth Street and Maine Avenue SE. One injured firefighter was transported from that location by ambulance.

A young black man had a seizure. Thank God the firefighters had made it on to the train by then. We were trying to escape through the back A video posted by Adjoa, Inc © (@adjyb) on Jan 12, 2015 at 2:15pm PST

According to D.C. Fire and EMS, 84 patients were taken to hospitals. Three are in critical but stable condition, two of them at George Washington University Hospital. More than 200 people were evaluated.

"There was a woman who was in distress on that train, and I'm sorry to say she's passed away," Metro General Manager and Chief Executive Officer Richard Sarles said.

The woman's cause of death and identity remain unknown.

"We are all saddened by today's fatality aboard the Metrorail, and our thoughts and prayers go out to the family of the passenger who passed away," a statement from D.C. Mayor Muriel Bowser read.

L'Enfant Metro Evacuated Over Smoke

Though an exact cause of the smoke and fire is unknown, NTSB investigators said there was an electrical arcing involving the third rail and the power supply cables to that rail prior to the fire. There was water along the tracks near the accident scene, which would have helped facilitate smoke along the third rail, investigators said.

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"It was just very smoky in there," passenger Fitz Carter said from aboard a Metro bus while waiting to be taken to a hospital. "We couldn't see anything. The visibility was poor. Right now, there's a lot of burning in my throat and my chest. Everybody got as low as they possibly could to the ground, because that's where the best possibility for oxygen was."

The FBI is assisting Metro Transit Police in determining a cause, which is standard procedure for incidents in the capital region.

News4’s Jackie Bensen reports live outside L’Enfant Metro station Monday afternoon, as rescue crews helped dozens of riders out of the system’s tunnels.

Green Line trains are operating on a normal schedule Tuesday. Blue Line trains will replace the Yellow Line from Huntington to Largo Town Center; trains will run every 12 minutes.

News4’s Scott MacFarlane has the latest on the closure of the L’Enfant Metro station in downtown DC, after reports of smoke in the tunnels.

Sarles is set to retire Friday as Metro's general manager.