It was a commuter’s worst nightmare: a Metro train abruptly stops, goes dark and fills with smoke in a tunnel in downtown Washington. Before it was over, one woman was killed and more than 80 passengers were suffering from respiratory problems and other health issues.

But a day after Monday’s ordeal aboard a Virginia-bound Yellow Line train, commuters returned to the rails.

"I have no fear of getting on the train this morning," said Martin Cullen, 37, a Jessup resident who works in data collections at the Smithsonian's American History Museum, as he walked to the Greenbelt Metro Station on Tuesday. "I've been riding the train for years and this is the first time something like that has happened."

And authorities now believe they know why the train, which had just left the L’Enfant Plaza station, came to a halt about 800 feet into the tunnel. The National Transportation Safety Board said “an electrical arcing event” occurred about 1,100 feet in front of the train. The event filled the tunnel with smoke, the NTSB said.

The agency said the arcing involved cables that power the third rail. Arcing is often connected with short circuits and may generate smoke. There did not appear to have been a fire.

Passengers were removed from a Yellow Line Metro train Monday afternoon after it filled with smoke. A rider reports that the train operator said, "there’s a problem, nobody move," then the car quickly began to fill with smoke. (Saleh Damiger/Jonathan Rogers)

[LIVE UPDATES: Follow the latest on the aftermath on Tuesday]

Metro’s Green Line resumed normal service Tuesday. Yellow Line trains were replaced by Blue Line trains and shuttle buses operated from L’Enfant Plaza, which was reopened.

The situation began to unfold shortly before 3:30 p.m.

Eugene A. Jones, the interim chief of D.C. Fire and Emergency Medical Services, said firefighters did not immediately enter the tunnel to help the riders because they were not sure whether the subway’s electrified third rail had been deactivated.

But Jones said the delay was “nothing like” the length of time described by passengers.

“While the power may have been turned off to the track bed where the firefighters would go in to make rescues, they heard trains running on the lower level” of the station, Jones said. “So in their mind, they wanted to make sure that before they put people on the track bed that the power was actually off.”

Jones said, “Once we worked with Metro to ensure the power to the track bed was off, we made entry and made rescues.”

(Related: Morning after, commuters return to the rails)

Passengers said the darkened cars quickly filled with smoke. Several riders said as much as an hour went by before firefighters arrived and led them out of the cars, escorting them back to the station. In the meantime, while the passengers waited in the smoke-filled cars, many were choking and some lost consciousness, witnesses said.

Passengers were stricken, and some were panicked.

“The power went off. The train stopped. The lights went out. And the smoke appeared,” said passenger Jason Hill, 31. “It built up fast, man, until you couldn’t see anything. Eventually it was like we were basically out of air.”

In the darkness of the cars, some riders screamed, people tried to pry open the doors to escape and others were overcome by smoke.

“People were panicking,” said Adjoa Adofo, 30, who lives in the Navy Yard neighborhood. “We didn’t know what to do, and we weren’t getting a lot of information.”

Saleh Damiger, 43, and Sirwan Kajjo, 28, said they thought they were “going to die,” as Damiger put it. “People were choking,” he said. “People were yelling. We couldn’t see each other. One woman, she started to pray.”

They said the train operator announced to passengers that they should stay low to escape the smoke while they waited to be evacuated.

“They told us to get down, get down [on] the floor,” Kajjo said. “Of course, it was dark, too. We couldn’t see.” But he and Damiger said they did see two people who appeared to be unconscious.

Matt Brock, a spokesman for George Washington University Hospital, said 34 people suffering from inhalation were taken there. He said their conditions varied: Most were moderately to mildly ill. Dozens of others were taken to Howard University Hospital and MedStar Washington Hospital Center, officials said.

“We have one fatality, a woman who was in distress on the train, which I’m very sorry to report,” said Metro General Manager Richard Sarles, speaking to reporters outside the station Monday night, his voice somber. “And my heart goes out to her family.”

The woman’s identity was not immediately made public as officials sought to notify her next of kin.

Sarles, who will soon retire, became Metro’s chief executive after a Red Line crash in June 2009 killed eight riders and a train operator. Monday’s incident was the first involving a passenger death since the 2009 crash.

Five subway lines — the Orange, Blue, Silver, Green and Yellow — pass through the L’Enfant Plaza station. The Yellow and Green lines run through an upper level. Those lines were shut down during the incident.

The Orange, Blue and Silver lines, which pass through the station at another level, continued operating, but trains on those lines did not stop at L’Enfant Plaza during the incident. Service to the station on those lines resumed.

There was no indication Monday when Yellow and Green line service would be restored to L’Enfant Plaza.

As the stoppage rippled throughout the system Monday night, passengers in stations served by the five lines milled about in stations, complaining that Metro had provided no information on why the trains were delayed.

Aboard the smoke-filled train, a young man appeared to suffer a seizure and an older man began banging on the doors while screaming profanities, witnesses said.

Then the train operator came on the announcement system to say there was no fire, just smoke, a woman said.

“That calmed people down a little bit,” Adolfo said. “But smoke continued to come in. The driver told us not to open the doors. That was the big thing. More smoke would come in. But people were panicking. They were trying to open up the doors anyway.”

“It started to get scary pretty quick,” said Jonathan Rogers, recalling smoke seeping through the doors.

He said the train operator told them of a plan to get back to the platform of the L’Enfant Plaza station, but that effort apparently failed.

“People started praying,” he said. “Smoke was coming in pretty steadily. Some people were fine, and some people were just hurting pretty quickly.”

He said a man standing next to him started having breathing problems and sank to the floor. Other passengers who grew short of breath passed around inhalers. They also shared water bottles.

A Metro Transit Police officer on one car told passengers, “If you need to hold somebody or you need someone to hold you, just turn to the person next to you,” said Allyson Burns.

“There was a lot of hugging,” said Burns, 36. “One woman sitting next to me put her head on my shoulder.”

Rogers said that when a woman beside him passed out, he joined other riders in trying to revive her with CPR.

“We just kept doing it, maybe 25 minutes,” he said.

Rogers estimated that it was 40 minutes before the rescue teams reached the stranded train cars. Using flashlights, rescuers led coughing passengers from the train, through a tunnel that had the stink of a chemical fire combined with the smell of burning wood.

They emerged from the station in the pelting rain to find a mass of emergency vehicles lining three blocks of Seventh Street SW, which was ablaze with strobing lights.

Rogers, who works for the D.C. Department of Transportation, took a Capital Bikeshare bike back to his office near the Navy Yard.

Burns took a bus to Rosslyn, hoping to reboard a train at another Metro station. But when she found the Metro too crowded there, she summoned an Uber car.

“I was like, just get me out of here,” she said.

Adofo emerged nearly in tears and said she prayed — Hail Marys and Our Fathers. “I’m just glad that I’m out of there.”

Ashley Halsey, Lori Aratani, Paulina Firozi, Martin Weil, DeNeen L. Brown and Victoria St. Martin contributed to this report.