It took Metro seven minutes to call 911 after a train stopped because of heavy smoke, and even then, officials did not report to emergency responders that passengers were trapped in a tunnel, according to federal safety investigators and reports from city officials and firefighters.

When the first rescuers reached the platform at the L’Enfant Plaza Metro station, they had no idea that hundreds of Yellow Line passengers were gasping for breath, according to internal District documents.

Knowing the severity of the situation Monday “would have made a world of difference” in getting more firefighters more quickly to the station, said D.C. Fire Lt. Stephen Kuhn, who led the unit that arrived first at the scene.

When Kuhn and his team walked into the tunnel, smoke was so thick that they could not see the red tail lights of the train “until we were right up next to it,” he said.

But a preliminary report released late Friday by the National Transportation Safety Board, which is investigating the incident that killed one passenger and injured 86 others, indicates that Metro understood how serious the incident was, even if officials didn’t report it that way. It took Metro only one minute after the train stopped to activate ventilation fans from its control center, showing that officials recognized that it was urgent to clear out the dense smoke.

The National Transportation Safety Board said Tuesday the smoke in the Metro tunnel near L'Enfant Plaza was caused by electrical arcing, which occurs when electricity escapes its prescribed path. Here's how that happens. (Jason Aldag/The Washington Post)

The preliminary NTSB report did not address the response to the emergency. But it did reveal that a second train was at the platform between the stopped train and the station. That would help explain why the stopped train could not back into the station to return and let passengers off.

The report also says for the first time that the train stopped at 3:15 p.m., a crucial element of the timeline. The fans were activated at 3:16 p.m. But according to a timeline previously released by city officials, Metro did not call 911 to report the smoke until 3:22 p.m.

In addition, the District’s timeline shows that emergency officials first became aware of people trapped via a cellphone call to 911 from a passenger at 3:33 p.m. A report of smoke or fire in a Metro tunnel triggers the dispatching of more equipment and closer communication with Metro’s operations center.

Carol Glover, 61, a federal contractor and grandmother from Alexandria, was killed by the smoke. The federal report said 86 passengers were taken to hospitals. Previously, officials had said that 84 people were hospitalized.

Metro declined to comment Friday about the preliminary report and has not made any statements about the incident, citing the ongoing federal review. District officials have released early accounts of its emergency calls, and Mayor Muriel E. Bowser (D) said officials would release their own preliminary report Saturday.

Videos taken by passengers inside the train and also on the upper levels of the station show smoke billowing through the cavernous and grand station. Photos released as part of the federal report show what was happening below: A meltdown that left huge gaps along the third rail.

It appears the incident was triggered by a malfunction in a power cable supplying electricity to the third rail, the NTSB said.

The report also says that a portion of the third rail remained live with power until 3:50 p.m. before Metro shut it down. Federal officials did not address whether the lag in shutting the power delayed firefighters. It also does not address communication problems among firefighters.

Kuhn said “it is possible” that the rail was live when he walked in, but “I have no way of really knowing.”

His rescue unit pulled up to the L’Enfant Plaza station at 3:31 p.m., and the firefighters immediately noticed an eerie site: Nobody was rushing out.

Kuhn said he and four other firefighters made it to the lower level in about five minutes and were immediately met by a group of Metro Transit Police officers. They “told us there was a full train in the tunnel with passengers.” One Metro officer was on a phone with Metro’s control center and told Kuhn that third-rail power was off and it was safe to start rescues, Kuhn said.

“When the cops told me power was down, power was down,” said Kuhn, 52 and a 29-year veteran. “That’s good enough for us and we went in,” about 3:40 p.m., Kuhn said. He said he triggered switches along the tunnel wall that are designed to cut power to the third rail for about 800 feet in either direction.

Kuhn said he was unable to communicate with fire commanders above ground to tell them that his unit was entering the tunnel, or even that people were trapped, because his radio wouldn’t broadcast from the tunnel.

He said his inability to radio his chiefs about power to the third rail may have contributed to later confusion.

It also may have delayed additional emergency units from entering the tunnel.

Federal investigators said their inspection found “severe electrical arcing damage” to the rail and cables inside the tunnel.

The back of the stopped car was inside the tunnel, only 386 feet from the platform at L’Enfant Plaza, the federal report said.

The second car at the station was at the platform about 500 feet behind the stuck train, according to the NTSB report.

Passengers aboard the train in the tunnel were unable to open the doors, so the firefighters used special keys they carry to get to them, Kuhn said.

He said the first person his unit encountered on the train was Glover, who was unconscious and had been carried two or three cars to the front of the train. A firefighter carried her on his shoulder out of the tunnel and to the upper level, where she received CPR for more than 10 minutes.

Elected leaders from across the region have demanded answers from Metro officials, saying that the hundreds of thousands of riders who depend on the system every day deserve assurances that their commutes are safe. On Wednesday, members of the region’s congressional delegation are scheduled to be briefed by NTSB officials on the status of the investigation.

“The facts and photos released today by the NTSB show that the Yellow Line incident on Monday was very serious,” Sen. Barbara A. Mikulski (D-Md.) said in a statement. “I appreciate the NTSB accelerating this preliminary report. But it’s just that — very preliminary. Many questions remain unanswered.”

The L’Enfant incident has raised fresh concerns about emergency procedures and maintenance practices at the transit agency at a time when the authority is in the midst of a leadership transition.

Friday was Metro General Manger Richard Sarles’s last day at the authority. An engineer by trade, Sarles has been praised for taking charge of a system that was reeling in the aftermath of a fatal 2009 Red Line crash and criticism of its lax safety culture.

Longtime Metro veteran Jack Requa will serve as an interim general manager until a permanent leader is chosen.

Kuhn praised the passengers for being calm amid the rescue.

“There was no pushing, no shoving,” he said, adding that he watched people make way for two pregnant women, as well as children, elderly and the infirm. He said the entire train was evacuated of several hundred passengers within 45 minutes.

Paul Duggan and Clarence Williams contributed to this report.