State transportation officials are probing why a budget estimate for the MBTA Green Line extension project was off by $1 billion.

As the state looks to figure out what went wrong with planning a long-awaited Green Line extension, the MBTA’s control board could be asking for outside help from consultants.

The MBTA’s fiscal and management control board Monday aired concerns about costs for the project, which would expand Green Line access through Somerville and into Medford. The budget for the project could be more than $1 billion higher than officials thought, we learned earlier this month.

How a project’s price tag could grow that much in such a short time - it increased to $3 billion over the latest estimate of about $2 billion – is a puzzling question officials hope to answer by tapping third-party experts.

Related: MBTA: No more folding $1 bills

“One of the things I’m perplexed by is that this was vetted by federal agencies, a number of different agencies,” said board member Brian Lang. “It’s mind-blowing to me that so many looked at it and said, ‘Yeah, this makes sense, the numbers line up,’ and for it to be so far off.”

The cost estimate for the project grew as design phases moved forward - from $953,000 in 2010, to $1.4 billion in 2013 and just shy of $2 billion in 2014, T officials said in a report.

State Transportation Secretary Stephanie Pollack said MassDOT would be responsible for paying the cost of hiring consultants.

On the table is whether there was a management failure behind the cost spike, she said.

To see costs rise so sharply likely means “something broke down somewhere along the line,” said Joe Aiello, the board’s chairman.

Related: Green Line extension in question after price tag jumps $1 billion

Transportation officials previously said cost increases were a result of a few factors. One was that, according to Pollack, construction costs were artificially low because they were budgeted based on prices during the economic downturn, The Boston Globe reported.

Options for the MBTA include slimming down some of the features of the project, taking funds from other planned projects and asking other stakeholders to help pay – The City of Somerville, for example, or Tufts University, according to a Globe analysis. Also a possibility is canning the project altogether.

Hanging over everything is a promise from the federal government to pay $1 billion to help complete the project - which is an opportunity the stat does not want to miss – the threat of legal action from the Conservation Law Foundation if the state fails to extend the line and the fact that the state has already spent about $300 million on the extension.

As officials parse the history of planning the project, several board members said “confidence” in the T to budget for projects is at stake. Without a good diagnosis of what went wrong and without the trust of taxpayers, they said, the much-anticipated Green Line extension could be a no go for Massachusetts.

“I don’t think that we can justify any decision going forward without giving a full explanation of how we got here in the first place,” said board member Monica Tibbits-Nutt