An investigation into the cause of smoke that killed one person and injured dozens on a D.C. Metro train could take six to 12 months, a National Transportation Safety Board spokesman said Tuesday.
Metro also identified the passenger who died as Carol Inman Glover, 61, of Alexandria, Va.
Eighty-four patients were taken to hospitals and more than 200 people were evaluated after smoke filled a subway tunnel and L'Enfant Plaza Metro station on Monday.
As of Tuesday evening, two passengers were still hospitalized at Washington Hospital Center, one in serious condition and one in fair condition, a hospital spokesman said. Four patients had been released.
According to Metro, seven patients were still being treated at George Washington University Hospital. All patients taken to Howard University Hospital had been treated and released, the agency said in a statement.
Tom Downs, chairman of the Washington Metropolitan Area Transit Authority Board of Directors, offered condolences to the family of the passenger who died, as well as an apology to Metro riders.
"Please know that once the cause of this incident is understood, we are prepared to take the actions needed to prevent this from happening again," he said in a statement. "The safety of each and every Metro rider and employee remains our absolute highest priority."
The NTSB is investigating the incident, which started about 3 p.m. Monday. It was caused by an electrical problem, NTSB investigator Mike Flanigon told reporters Monday night.
A southbound Yellow Line train stopped about 800 feet beyond the platform at the L'Enfant Plaza station, Flanigon said. About 1,100 feet beyond where it halted, "there was an electrical arcing event" involving the electric third rail that powers the train and supply cables leading to the rail, he said.
The tunnel was filled with smoke and passengers got out of the train by themselves, Flanigon said. The train did not derail and there was no fire.
NTSB spokesman Peter Knudson said the agency was working to provide an update on the investigation within a week.
"What we're focused on now is establishing the facts and circumstances of the accident to get into the cause," Knudson said. "That is what will come out at the conclusion."
The NTSB investigation is the latest into incidents on the transit system since 2000.
In 2004, a train rolled backward and struck another train, injuring 20 people. The NTSB also investigated two incidents in 2006 in which trains struck and killed several Metro employees.
In 2007, it investigated a derailment that injured 23 people.
The deadliest wreck in the transit system's history came in 2009, when a train slammed into a stopped train, killing nine people and injuring dozens. The NTSB determined the probable cause of the incident was a failure of track circuit modules and the system's inability to ensure that the enhanced track circuit verification test was used system-wide.
That same year, the NTSB investigated a collision between two out-of-service trains in which three employees were injured. In 2010, the board investigated two accidents, one that resulted in the death of two workers and a derailment that injured three passengers.