A member of the Federal Communications Commission has added his voice to the growing chorus of people urging Metro to accelerate its effort to expand cellphone service throughout the subway system for the sake of riders’ safety.

In a two-page letter to Therese W. McMillan, acting administrator of the Federal Transit Administration, Commissioner Michael O’Rielly characterized the lack of reliable cell service in the nation’s second-busiest subway system as a “glaring safety problem.”

“Respectfully, I implore you to consider that passengers and workers using their own wireless devices throughout the tunnels and stations to convey critical real-time information in emergency situations are the first line of safety,” O’Rielly wrote.

“Quite simply, when D.C. Metro riders — often the first to see a problem developing — try to notify first responders, they frequently are unable to receive a signal strong enough to make a simple call to 9-1-1 to report the emergency,” O’Rielly said.

WMATA surveillance video inside L’Enfant Plaza Metro station on Jan. 12, 2015, shows people on the platform evacuating, as smoke can be seen coming from the southbound tunnel for the Green and Yellow Lines. (NTSB) VIDO SCREEN GRAB (NTSB)

Metro announced last month that it had reached a new agreement with four major wireless carriers on a plan to make service available throughout the transit system’s 101 miles of tunnels by the end of the decade.

The original contract had called for the four companies to install the necessary cables in the tunnels, but the project has been mired in delays that probably would have continued. So, under a new plan, expected to be finalized soon, the consortium of carriers will pay Metro tens of millions of dollars to do the tunnel wiring. That prospect concerns some observers, who cite Metro’s troubled performance record.

O’Rielly said that the FTA should use its newly expanded safety oversight role — announced earlier this month — to ensure Metro does the work properly and in a timely manner.

“The Federal Transit Administration’s recently-asserted safety oversight responsibility for Washington Metropolitan Area Transportation Authority’s transit rail operations provides an opportunity to reverse flawed decisions and reenergize efforts to deploy commercial wireless infrastructure within the system,” O’Rielly wrote.

While the FCC has no immediate authority over Metro, O’Rielly noted that the commission’s work includes ensuring the nation has a reliable emergency communications system.

FTA spokeswoman Angela Gates said the agency had received the letter and would respond at a later date. Metro officials referred calls to the FTA.

Even before the fatal Jan. 12 smoke incident killed one person and sickened scores of others trapped aboard a Yellow Line train outside the L’Enfant Plaza station, the lack of reliable cell service in the system has been a ­long-standing concern among riders and first responders. A series of safety failures and service breakdowns in recent months has only heightened worries.

Cellphone service has long been available on Metro’s underground platforms, with varying reliability, but service in tunnels has been a different matter.

Under a 2008 contract between Metro and a consortium of Verizon, AT&T, Sprint and T-Mobile, the effort to bring reliable cellphone service to Metro was scheduled to be completed in 2012. But Metro officials said several obstacles, including unforeseen logistical hurdles, a bankruptcy and the deadly 2009 Red Line crash, forced it to give higher priority to safety-related infrastructure work in the system that prevented the companies from finishing the work.

The delays have frustrated riders — and caught the attention of Congress, including Rep. John L. Mica (R-Fla.), who threatened to seek a reduction in federal funding for Metro if the cell service project continued to be delayed. Mica said last month that he was “satisfied” with the pending agreement.

But others said Metro must complete the work sooner.

On Thursday, Prince George’s County Fire Chief Marc S. Bashoor, who briefed members of the Metro board’s Safety Committee about ongoing work to upgrade emergency radio communications in the transit system’s tunnels, said the agency must make the project a priority. He said he understands the logistical challenges the transit agency faces.

“Part of the issue is, you have live tunnels that are very narrow areas of operation,” Bashoor said. “So it’s not that they can just shut down the tunnels and run the cables through. With the service that’s provided except for four hours a day, there’s not enough time.”

Nevertheless, he said, “five years to get cell service implemented is frankly unacceptable. . . . Getting it done is important to us. Getting it done as quickly as possible is the most important thing.”

The project is expected to begin in January, and Metro would use equipment provided by the consortium companies, officials have said.

The FCC’s O’Rielly called the project timeline “troubling” and said it should inspire the FTA “to examine ways to expedite the work.”

“In sum, FTA should establish milestones and associated deadlines to ensure WMATA’s work is conducted successfully and as quickly as possible,” he said.

Even with its expanded oversight role, it’s not clear whether the FTA could require Metro to expedite the installation of wireless service. Cell service was not examined as part of the FTA’s recent safety review of the transit agency.

But Bashoor said reliable service is a key to keeping people safe. “For us, it’s a problem of public safety. We have to make sure people have access to 911.”

Paul Duggan and Brian Fung contributed to this report.