WASHINGTON -- Passengers on a smoke-filled subway train in the U.S. capital were still asking when help would arrive 27 minutes after the smoke was first reported, city officials said Thursday.

One woman died and dozens more were sickened when the train filled with smoke Monday afternoon near a busy station in downtown Washington. Some were vomiting, others were choking and gasping for air.

"There was no oxygen," one woman told CBS News. "People started panicking and people started hitting the floor."

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The cause of the electrical malfunction that led to the smoke remains under investigation.

Officials released a timeline of the emergency response Thursday morning. While it does not disclose the time that paramedics reached the smoke-filled train, it confirms the accounts of passengers who said they waited more than a half-hour for help.

The first report of trouble came at 3:18 p.m., when a 911 caller reported smoke emerging from a tunnel near the station. Several 911 calls followed, and at 3:45 p.m., two people called from the train to ask whether help was on the way.

It was another 24 minutes after that, at 4:09 p.m., that officials reported performing CPR on the woman who later died. Passengers have said they tried to revive her on the train before emergency responders got there. She wasn't taken to a hospital until 4:25 p.m. - more than an hour after the smoke was spotted.

The timeline also shows that firefighters were on the scene for 13 minutes before Metro subway officials confirmed that people were trapped on a train and that the electrified third rail had been shut down.

The first reports of trouble actually came from another downtown Metro station about a mile( nearly 2 kilometers) away. According to the timeline, Metro called to report a debris fire on the tracks. It was not clear whether that fire was related to the electrical malfunction that caused the smoke, but it occurred north of the L'Enfant Plaza station near the Air and Space Museum.

Many passengers on the stricken train decided to evacuate on their own before emergency responders got there, despite instructions from the train operator to stay put.

"We were not given any information that police or fire were en route or nearby," said Luis Clemens, 47, a National Public Radio editor who left the train. "All we got was, 'Stay in place. Yes, I know there's smoke. Don't leave.' And that doesn't make a whole lot of sense when you're sitting there, watching, over some period, watching the subway cars fill up with smoke."

The Metrorail system, which connects Washington with the Maryland and Virginia suburbs, carries an average of 721,000 passengers each weekday.