The Massachusetts Bay Transportation Authority has received a tentative federal funding commitment for nearly $1 billion, almost half the cost of constructing the Green Line extension project. Congress must approve the grant.

In a Dec. 1 letter to Congress, Therese McMillan, acting administrator of the Federal Transportation Administration, announced plans to "execute a full funding grant agreement," totaling $996 million for design and construction of the 4.7-mile Green Line extension. The grant would include the construction of six new stations to carry trains into Somerville and, eventually, Medford, beginning in the next five years.

The federal grant funding is expected to cover 43% of the MBTA's overall $2.2 billion in project costs, with state operating funds and general-obligation bonds covering the remainder. The FTA and its financial management contractor have determined the MBTA has the "requisite financial capacity" to finish the project and operate and maintain the system once it is up and running, the letter also stated.

MBTA General Manager Beverly Scott said the announcement is "terrific news" for Green Line riders. "With Gov. [Deval] Patrick's [D] unwavering support and a strong grant application, we convinced the FTA that extending the Green Line is among the most important transit projects in the nation. I believe this process has been a model for demonstrating how local, state and federal officials can work together to achieve greatness."

An FTA spokeswoman said the administration was not at liberty to comment on the funding agreement since it was under a 30-day congressional review, ending on Dec. 31.

Barletta Heavy Division began work on the $20-million first phase in 2013. A White-Skanska-Kiewit joint venture is construction manager and general contractor for the next three phases.