BOSTON (CBS/AP) — The price tag for the MBTA’s planned Green Line extension into Somerville and Medford could come in at up to $1 billion more than first estimated.

The cost to the state for the expansion was initially projected at about $2.3 billion.

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MBTA officials told reporters on Monday that the cost could balloon by an additional $700 to $1 billion, putting the future of the project in question.

“It’s still a great project with a lot of benefit, not just for Somerville but for the entire region, but we have to find an affordable way to build it or we can’t move forward,” transportation secretary Stephanie Pollack told WBZ-TV.

The 4.7-mile project calls for the construction of six new stations, the relocation of one existing station, and the purchase of new light rail vehicles.

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Somerville Mayor Joseph Curtatone says he’s not giving up on the project just yet.

“This is the best transit project in the books,” Curtatone said. “This is why the feds supported it, why the Governor sees the promise in it.”

State transit officials are looking at a range of options, from redesigning the project, coming up with additional funding, renegotiating with the contractor, or abandoning the extension.

Federal officials have already promised nearly $996 million to the project.

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