THE MBTA ON THURSDAY CANCELED THE CONTRACTS of the four major firms working on the Green Line extension and announced it was also going to shake up its own internal project management team.

The decision was made after the MassDOT board and the T’s Fiscal Management and Control Board went into executive session on Wednesday to discuss “litigation strategies” in connection with the project. The two boards appear to be severing ties with the contractors, employees, and contracting approach that produced a project that ballooned in cost from $2 billion to $3 billion and in the process became the most expensive project of its kind in the country.

On the Green Line extension, state law directed the T to use a new contracting approach called construction manager/general contractor, or CM/GC, which called for the hiring of a contractor, designer, and independent cost estimator to design the project and come up with a guaranteed maximum price. A consultant hired by the T told the boards on Wednesday that the process never yielded a guaranteed maximum price.

In a press release issued by the MBTA on Thursday, the transit agency said “the current CM/GC project delivery method has proven to be impracticable to complete a redesigned and affordable Green Line extension project. The current CM/GC contract and the statute that created this project delivery system combine to render the current process unworkable. Accordingly, the Fiscal Management Control Board needed to recommend the full break from CM/GC and the associated contracts for this project.”

The four major firms working on the project were the construction manager/general contractor, White-Skanska-Kiewit; the project manager/construction manager, HDR/Gilbane; the independent cost estimator, Stanton Constructability Services; and the final designer, AECOM/HNTB. The MBTA said Green Line construction work in progress by other firms will continue.