Ten days after scores of Metro passengers were stuck on a smoke-filled subway train, one aspect of the bewildering crisis became somewhat clearer Thursday as the transit agency explained why first-responders had trouble communicating by radio during the emergency. Yet broader questions about what caused the fatal incident remained unanswered.

Top Metro officials, speaking publicly for the first time about the Jan. 12 calamity, also addressed the question of whether the ventilation system worked properly in the subway tunnel near the L’Enfant Plaza station.

The acting head of the National Transportation Safety Board, who met with Metro officials Thursday, said that ventilation fans in the tunnel “weren’t functioning as intended” when federal accident investigators tested them shortly after the incident.

Rob Troup, Metro’s deputy general manager, acknowledged the potential problem with the tunnel fans and said the transit agency has begun an “immediate, full-scale investigation” of all 200 fans and 81 ventilation shafts throughout the subway system.

“It’s important for us to go out and look at all our fans to make sure they’re working properly,” Troup told reporters Thursday at Metro headquarters. “We’re currently doing that.”

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)

Still, many elements of the incident have yet to be explained, including whether there were broader communications problems between Metro and other agencies, why passengers were stuck on the train for at least 35 minutes as they waited for help, and why the Yellow Line train was unable to be moved as smoke filled its six cars.

On the issue of radio communication, Metro’s interim general manager, Jack Requa, spoke at length during and after a public meeting Thursday.

As rescuers worked to evacuate the choking train riders after an electrical meltdown on Metro tracks caused heavy smoke in the tunnel, D.C. firefighters below ground were unable to clearly communicate by radio with commanders outside. One of the passengers, Carol I. Glover, 61, of Alexandria, died of smoke inhalation.

Requa said that several days before the incident, the fire department changed the way its radio signals operate. But the department did not inform Metro of the change.

Metro’s subway system is equipped with signal-relaying equipment that normally allows firefighters to use their radios below ground. The radios did not function properly Jan. 12 because Metro had not altered its relaying equipment to conform to the changes in the fire department radios, Requa said.

As for whether fire officials should have informed Metro of the changes, so that the agency could make the needed adjustments to its equipment, Requa said: “Hopefully, we would cooperate in that manner. Maybe they thought the changes­ that they made would not impact communications.”

The fire department did not respond to a request for comment Thursday.

Smoke and fire incidents on Metrorail since 2012
While still sporadic, smoke incidents have decreased since peaking at 20 incidents in June 2012. Fire incidents have remained fairly steady, peaking at 13 incidents in May 2014.

(Denise Lu/The Washington Post/WMATA)

Mayor Muriel E. Bowser (D) said a forthcoming report from District public safety officials, which she said would be delivered no later than Friday, “will cover everything we know about communications.”

Bowser said she has not been told by either D.C. Fire and Emergency Medical Services or by the NTSB that the recent decision to encrypt FEMS radios was a factor in the communications problems first-responders encountered at L’Enfant Plaza.

Metro spokesman Dan Stessel said the transit agency was not engaging in “finger pointing.”

“We want to understand what happened at L’Enfant Plaza, learn from it and move forward,” Stessel said. “Every day, [D.C. firefighters] perform their jobs with skill and bravery, and it is in our mutual interest to ensure that firefighters have reliable communication so that they can work safely.”

After the fire department made changes­ to how its radios transmit, firefighters responded to a Jan. 7 debris fire at the L’Enfant Plaza station and discovered that their radios were not working properly inside the station. The fire department e-mailed Metro on Jan. 8, asking whether the transit agency could pinpoint the cause of the problem.

Requa told members of Metro’s board of directors Thursday that radio engineers for the transit agency spent the weekend of Jan. 10 and 11 trying to determine the cause of the problem. It is Metro’s responsibility to maintain the signal-relaying equipment in the subway for use by emergency personnel.

By Monday morning, hours before the riders were trapped on the smoke-filled train, Metro workers had “looked at everything possible . . . trying to sort out the radio trouble” on Metro’s side of the system, Requa said. He said the radio engineers were left with “more questions than answers.”

He said they then scheduled a meeting for Jan. 14 with D.C. fire officials to attempt to pinpoint the cause of the problem.

The fatal tunnel smoke incident occurred before that meeting could be held.

Requa said it was eventually determined that firefighters had changed the encryption codes on their radios and made other changes without notifying Metro. Once the confusion was discovered — at the Jan. 14 meeting — the problem was easy to solve.

“We went back and made the modifications that we needed to make to allow that system to work,” Requa said. “The fire department tested their radios and the system was up and running, and it’s running today.”

Meanwhile, among the many aspects of the incident that are under investigation, the NTSB is trying to determine whether the tunnel’s ventilation system worked properly,

There are two large ventilation shafts near where the train stopped. Ideally, the large fans in one shaft should have pulled smoke out of the tunnel and expelled it outside while the fans in the other shaft pulled in fresh air from above ground, said Christopher Hart, the NTSB’s acting chairman.

But when tested after the emergency, Hart said, the fans did not function correctly.

A big question for investigators is whether the fans did their job when the tunnel filled with smoke last week. “That’s why we need to find out what happened during the incident,” Hart said. “We don’t know yet. But we intend to find out.”

As for the cause of the smoke, federal investigators said their inspection found “severe electrical arcing damage” to the rail and cables inside the tunnel.

The electrical malfunction, known as “arcing,” occurred in a subway power line called a “jumper cable” and involved the electrified third rail, from which trains draw propulsion power.

In a tunnel, there are gaps in the third rail where ventilation shafts are located so that during an evacuation, people can get out through a shaft without the danger of stepping on the third rail. The gaps, commonly about six feet wide, are bridged by jumper cables, which are insulated power lines the keep electrical current flowing along the third rail.

If a jumper cable is deteriorated or damaged in some way, and electricity begins to escape, the phenomenon is called “arcing.” It can generate sparks, heat, melting and smoke.

While the investigations continues, Troup said Thursday that Metro plans to begin installing new “low smoke/low halogen” jumper cables in the subway and also “mechanical protection on third-rail jumper cables that may be exposed to wear from vibration against other materials.”

Photos released in a preliminary NTSB report showed jumper cables that were melted and burned.

“We know there was arcing,” Hart said. “What we don’t know yet is why.”

Mike DeBonis contributed to this report.