For scores of choking passengers caught in a smoke-filled Metro train Monday and waiting to be rescued from a tunnel, the ordeal seemed interminable. By some accounts, 40 minutes or more went by before firefighters showed up and forced open the doors. A lot of riders said they feared they would die. And one woman did.

On Tuesday, as the National Transportation Safety Board continued investigating the tragedy near the L’Enfant Plaza Metro station, District officials, public-safety authorities and the NTSB declined to comment on the specifics of what happened, including the timeline of how Metro and firefighters responded to the incident.

The NTSB, without offering details, said Monday’s incident was caused by an electrical malfunction in the tunnel.

D.C. Mayor Muriel E. Bowser (D), who also disclosed few specifics, said that after firefighters were alerted to the emergency, they began arriving at L’Enfant Plaza “within the time frames that are customary.” Yet for reasons not yet clearly explained, they waited in the station before entering the tunnel and walking 800 feet to the train.

One rider, Carol I. Glover, 61, of Alexandria, would soon be declared dead, and 83 other passengers, sickened by smoke, would be taken to hospitals.

A Metro passenger filmed the scene inside the stranded Metro car, in which people can be seen helping and comforting each other. A Yellow Line train abruptly stopped and filled with smoke in a tunnel in downtown Washington on Monday. (Saleh Damiger/YouTube)

For Metro, this wasn’t the first time in recent years that the need to evacuate riders from an immobile train led to confusion and delay.

In 2012, passengers on a disabled Green Line train pried open doors and walked along tracks toward the College Park station. Some claimed that the train operator had told them to do so; Metro disputed that. Less than a year later, nearly 2,000 riders were stranded on two Green Line trains. Frustrated by what they said was a lack of information, many of them walked along tracks after forcing open doors.

The transit agency — which has long had trouble with evacuations — promised in 2013 that it would review its protocols, conduct more training for employees and identify “best-practice models on train evacuation.”

Citing the continuing inquiry into Monday’s incident, Metro officials declined Tuesday to discuss the evacuation procedures that the agency had vowed to improve.

“All those questions touch on issues that will likely be part of the NTSB investigation,” Metro spokesman Dan Stessel said. “As such, we are unable to comment. Again, it is imperative that Metro not give the appearance of trying to influence the investigation in any way by commenting during the NTSB process — as required by the federal regulation.”

While Bowser, top Metro officials and others declined to discuss the investigation, Sens. Mark Warner (D-Va.) and Barbara A. Mikulski (D-Md.) as well as D.C. Del. Eleanor Holmes Norton called for more information and briefings from the NTSB.

“While the [NTSB] investigation is just getting underway and may require months to complete, Virginians are climbing back aboard Metro trains,” Warner said. “Passengers deserve to know as soon as possible about Metro’s safety protocols for this type of incident, and those answers should be provided right away.”

How arcing happens.

The first report that D.C. firefighters received about trouble in the tunnel came about 3:20 p.m. A Virginia-bound six-car train stopped suddenly in the darkness, the lights in the train went out and the cars began filling with smoke, passengers said.

They were choking, and some were panicked or lost consciousness as the train operator urged them to be calm, saying he would try to drive the train back to the L’Enfant Plaza station. Why the train stayed put — whether it was disabled or the driver wasn’t cleared by the station manager or command center to return to the station — is a question still to be answered.

As for the response by firefighters, District officials declined Tuesday to discuss specifics. Bowser said it is unclear whether firefighters thought they could safely enter the tunnel, where trains are electrified by a high-voltage third rail.

Whether Metro had turned off the third-rail power by then or whether it had been lost by some other means also remains unanswered.

Bowser said that she expects that the NTSB’s investigation will “really determine what Metro knew and when they knew it.” She added: “I think the key points will be: What time did we receive the calls? To what locations were we informed that there was help needed? When did our units arrive? And when did we have safe access to the tracks?”

She added: “We don’t know what time the train actually went down” — whether there was a time lag before D.C. firefighters were summoned about 3:20 p.m.

“This is the question that I know that will come out of the NTSB’s review, and we may be able to report it when our review is concluded as well,” Bowser said. “That’s one key piece of information that we don’t know.”

She pledged that D.C. officials would do a “deep dive” into the facts and issue a report by early next week on the city’s response.

A passenger, Sean Lynch of Arlington County, said that for at least 20 minutes, the train operator announced repeatedly that he would return to the station rather than wait for firefighters. But as time passed, that did not happen.

Lynch said the operator “went over the intercom and asked all WMATA staff on the train to call the station manager at L’Enfant Plaza to convey the severity of the situation because the manager would not authorize our return to the station.”

Lynch added: “He kept going over the intercom to say that we are about to move, and he tried three, four times — the train would shudder for a second like it was going to go and then would stop again.”

Andrew Litwin, 21, a University of Texas junior who was visiting relatives in Alexandria, had just finished a day of touring Washington.

He was in the rear car as the train left L’Enfant Plaza.

Then it stopped, and the lights went out. Looking out the windows from darkness, passengers could see haze in the tunnel. “We could hear people shouting from [the cars] in front of us . . . that there was smoke,” Litwin said. “We were a little bit panicked at first.”

The train operator walked through the cars to the controls in the rear car and said he would drive the train back to L’Enfant Plaza once he got the signal from Metro controllers.

There was no fire in the tunnel that Litwin could see. He said smoke began wafting into the cars, yet the train did not move.

The operator “asked everyone to remain calm and keep away from the doors,” Litwin said. Some passengers began to shout. Litwin said he heard someone yell: “Please, sir: Move the train! People are going to die here!”

Litwin said, “Some people were irate, but as the time passed, people realized they had to focus and breathe.” Riders held scarves, hats and shirts to their faces. Litwin said he prayed and said to himself, “This would be a terrible way to die.”

Still, the train did not move. Litwin said riders around him spoke of forcing open the doors and walking back to L’Enfant Plaza. “People were ready to go. People weren’t going to chance staying on that train too much longer.”

Forty-five minutes went by, Litwin said. Firefighters did not arrive.

Eugene A. Jones, the District’s interim fire chief, said there was an initial question over whether power to the third rail had been cut before firefighters went into the tunnel. He said some firefighters heard trains, apparently coming from a track on a lower level, and waited to confirm with Metro that no current was running through the third rail.

Jones declined to say whether fans to vent the tunnel were working.

The Office of Unified Communications, which runs the District’s 911 center, would not release the dispatch and arrival times of firefighters. The Fire Department also declined to provide a timeline of how the rescue unfolded.

The NTSB said the incident had been caused by an electrical “arcing event” that occurred about 1,100 feet farther into the tunnel from where the train stopped.

Louis Sanders, director of technical services for the American Public Transportation Association, said in an interview that “arcing” can cause a fire.

Sanders, who is not involved in the investigation, said the third rail, which carries the electricity that powers trains, is mounted on “insulators” that keep the rail from making contact with the ground. The electricity flows into the third rail through power cables connected to the rail at various intervals.

He said it is important to keep the electrical current, usually about 750 volts, contained within the power cables and the third rail. However, sometimes the electricity escapes, which is called “arcing,” which usually happens in a flash.

There is also “a more insidious” problem: Stray current, Sanders said, which is when electricity escapes not just for an instant but continuously.

Electricity could escape from deteriorated cables or from the third rail if the insulators keeping the rail off the ground are faulty. Once the electricity starts flowing uncontained, it would generate “tremendous heat,” possibly resulting in a fire.

Such an electrical malfunction could be related to how well the cables, insulators and third rail have been maintained, Sanders said.

In 2013, Metrorail had 86 fire and smoke incidents, not all in tunnels, according to the agency’s safety reports. Last year, there were 85 such incidents in the first eight months, the most recent reports show.

Monday’s passenger fatality was the first in the Metro system since eight riders and a train operator died in the 2009 Red Line crash. General Manager Richard Sarles, who is set to retire this week, was brought in to lead the agency after the disaster, which the NTSB attributed partly to a lax safety culture within Metro at the time.

Sarles has said that as the top official of the nation’s second-busiest subway, improving the agency’s safety culture has been one of his biggest accomplishments.

Bowser, who was a Metro board representative from 2011 until she was elected mayor in November, agreed with Sarles, saying that Metro safety has “dramatically improved.”

“That’s why it’s shocking and so disappointing that we’ve had this failure.” she said.

Lori Aratani, DeNeen L. Brown, Pamela Constable, John Woodrow Cox, Mike DeBonis, Paulina Firozi, Ashley Halsey III, Dana Hedgpeth, Joe Heim, Justin Jouvenal, Michael E. Ruane, Katherine Shaver, Lena H. Sun, Robert Thomson and Julie Zauzmer contributed to this report.