Last week, the MBTA revealed that the project to extend the Green Line into Somerville and Medford could be as much as $1 billion over budget. Now, the panel tasked with fixing the T's finances wants to know how estimates could have been so far off.

The MBTA's Fiscal and Management Control Board is Gov. Baker's hand-picked team put empowered to figure out how to make the T fiscally sound. And after it was revealed last week that the Green Line project could cost about fifty percent more than expected, the panel now wants to hire an outside consultant to analyze the cost increases.

Board member Steven Poftak wants a neutral third-party authority to answer why construction costs rose so dramatically between January and May of this year and even if the current projections can be trusted.

"What went wrong here and what changed so radically from the beginning to the end, I think, is really important for us to have any confidence in this going forward," Poftak said. "I'm not wholly confident that we've got this right yet. I'm not wholly confident that the last set of estimates we have is in fact the end point, the correct set of assumptions."

Poftak and his fellow fiscal controllers asked that the outside group be free of conflicts and experienced dealing with the kinds of cost estimates at play.

After hearing the questions the panel want answered, Transportation Secretary Stephanie Pollack will hire an outside group to study what happened and report back.