Metro acknowledged Tuesday that it has blown the deadline to have cellphone service available throughout its rail system -- and won't be able to finish the work until December 2015.

The transit agency wrote in a letter to the House Oversight and Government Reform Committee that it was not done with the work required to be finished Tuesday under a 2008 federal funding act that allocates $1.5 billion to the transit agency. The Washington Examiner first reported last month that a delay was expected.

Metro said 44 of its 47 underground stations have wireless service, but the connecting lines in the underground tunnels are not complete.

"Despite our considerable progress, there is still significant work to be done to ensure wireless access throughout the entire system, which will require a level of coordination and logistical planning rarely seen in the transit industry," General Manager Richard Sarles wrote. "Moreover, we are further challenged by the fact that the work must take place as we also continue our highest priority of making urgent safety upgrades."

Metro's federal funding is not in jeopardy yet, though. The agency was given a reprieve until March 27 by a congressional resolution last month. And local lawmakers plan to push for an additional extension.

Metro blamed the delay on the investigation into the deadly 2009 Fort Totten crash. The agency also said the project required "significantly more time than any of the parties envisioned" when the legislation was drafted in 2008.

But it appears work has slowed in the past year.

In an October 2011 report to Congress, Metro said it had 82 percent of stations done. Today, it has wired only two more stations. In the first year, it wired the 20 busiest stations. In the subsequent two years -- even in the aftermath of the deadly crash -- it completed an average of 11 per year. Metro declined to comment on the slowdown or why it has not told riders directly about the delays. The agency referred questions to the cellphone carriers installing it, who did not respond Tuesday afternoon.