An investigation has been launched to determine how and why black smoke filled the L'Enfant Plaza metro station in downtown Washington D.C. on Monday, killing one person and hospitalizing 84 others.

The incident occurred Monday afternoon when a Washington D.C. Metro train abruptly stopped in a tunnel and went dark, while smoke filled six subway cars, reports CNN.. As a result, one woman was killed and more than 80 passengers were sent to local hospitals after suffering from respiratory problems and other health issues; two people were critically injured.

Because the incident happened at the L'Enfant Plaza station, a key transfer point for five Metro lines, there were major disruptions on other lines.

The National Transportation and Safety Board released a statement saying "an electrical arcing event" sparked the incident.

Peter Knudson, a spokesman for the National Transportation Safety Board, said in a statement that about 1,100 feet ahead of the train ""there was an electrical arcing event involving the trackside power cables," or the third rail.

"Passengers on the train self-evacuated.," Knudson said. "The train did not derail. There was no fire on the train."

According to Reuters, Knudson added that "what caused the electrical arcing" is what his team hopes "to understand." Officials say arcing occurs when electricity from the power rail makes contact with something that conducts electricity, like water.

During the investigation, which is expected to take six to 12 months, authorities will look into the emergency response, the statement said.

According to the Washington Post, some passengers said it took firefighters an hour to reach the train.

Tom Downs, the chairman of the Washington Metropolitan Area Transit Authority, issued a statement apologizing for the incident and promising that Metro will do all it can to prevent another one once the investigation is completed.

By Tuesday, Metro was running with delays on five of six lines.

"I have no fear of getting on the train this morning," commuter Martin Cullen told the Washington Post on Tuesday. "I've been riding the train for years, and this is the first time something like that has happened."

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