The MBTA's failure to follow best practices for a new contracting process was one of the major factors responsible for driving up the cost the Green Line extension project, consultants told state transportation officials on Monday.

The Berkeley Research Group (BRG) was hired to find out how the estimated cost of completing the Green Line extension to Somerville and Medford swelled from about $2 billion to nearly $3 billion in less than a year. The firm presented its findings to a joint meeting of the T's fiscal control board and the state Department of Transportation board on Monday.

According to the analysis, the T failed to properly implement a new procurement method, which was being used by the state for the first time. As a result, the contract that was negotiated with White Skanska Kiewit (WSK), the group of primary contractors working on the project, failed to require the reporting of itemized project costs, broke the project down into too many separate parts, and enabled WSK to negotiate to secure the highest price allowed by the contract for its work.

"WSK figured out how to work the system, we allowed them to work the system, and they ended up every time getting their maximum price," Brian Lang, a member of the T's fiscal control board, said during Monday's meeting. "I think that's fair," BRG's Terry Yeager responded.

The analysis also found that the equivalent of just four full-time T employees were dedicated to overseeing the project — too few for a project of this size, BRG said. The agency instead relied on outside firms to step in and be "used as additional MBTA staff."

That was a missed opportunity for the T to gain institutional knowledge about this new procurement process, according to BRG's Terence Rodgers, and meant there wasn't enough internal focus on how the project was performing.

"No one is going to be concerned about the owner's project any more than the owner," Rodgers said. "We had some very fine and talented and great professional service providers involved in this, but at the end of the day the owner is going to be the most concerned about their progress, their performance and their risk."

The T was also never able to come up with a reliable project cost, something BRG's Yeager said the T should have been able to do as early as 2012. "I'm not convinced that a reliable budget has been produced to date," he said.

The future of the Green Line extension has been in question since officials revealed in August that it would cost an estimated $3 billion to complete the project, $1 billion more than was estimated in 2014.

While construction on parts of the project continues, other aspects have been paused as the state figures out how or whether to move forward.

BRG's analysis emphasized that there was no "silver bullet" problem with the project, but that several longstanding and connected issues at the T led the project to this point.

"It seems that many parties failed to undertake actions that would have helped us much better understand the cost of this project," Transportation Secretary Stephanie Pollack said after the meeting. "The cumulative result is that we are standing here having already [approved] four construction contracts without actually understanding what it's going to cost to get to the end of the project."

State officials are planning to discuss next steps for the project at a Dec. 9 meeting.