Investors behind a $1.5-billion downtown Boston development project could find themselves on the hook for the cost of a sudden, unplanned three-day shutdown of two key subway tunnels.

The Massachusetts Bay Transportation Authority (MBTA) on June 26 reopened the Orange-and-Green line tunnels underneath the massive government center garage, which is in the middle of a multi-year demolition and redevelopment project by HYM Investment Group.

The MBTA was forced to close the same tunnels in March after the sudden collapse of a section of the garage during demolition work sent rubble tumbling to the ground above the tunnels.  A 51-year-old heavy equipment operator was killed and the tunnels underneath the garage were closed for several days.

 The MBTA is now blaming real estate giant HYM for the agency’s unexpected decision on June 23 to shut down the subway tunnels underneath the old garage, disabling a key piece of downtown Boston’s transportation system.

In a statement, officials said they made the decision after being informed by HYM that one of the garage’s support columns, which extend down through the tunnels underneath the structure, is “severely deteriorated.”

MBTA General Manager Steve Poftak wasted no time pointing the finger at HYM, contending the real estate investment firm, headed by former City Hall development chief Tom O’Brien, would be on the hook to pay for “all costs associated with the event.”

A spokesperson for the MBTA reiterated the agency’s stance in an email in response to a question from ENR.

“The MBTA is in the process of determining the financial impact this will have and will hold HYM fully accountable,” the spokesperson wrote.

But HYM, which has been taking down sections of the garage to make way for new towers and buildings as part of its Bulfinch Crossing megaproject, has argued that the latest shutdown had nothing to do with the construction work on the garage site.

Rather, a “subsurface column” near the Green and Orange line tunnels was “identified as compromised from years of water damage,” HYM and National Real Estate Advisors said in a statement.

In addition, the column’s condition is “unrelated to the demolition work at the Government Center Garage,” the project’s development team contends.

HYM has not responded to additional questions and requests for information from ENR since sending a statement out on June 24. 

Still, since the June 23 shutdown, the developer “has installed the necessary supports to uphold the structure,” the MBTA said in a statement.

Structural engineers and safety experts conducted “comprehensive inspections” of the repair work, deeming “structures and the surrounding infrastructure to be structurally sound,” the T said.

Test trains were sent through the tunnels before passenger service was restarted, the authority said.

Still, questions remain as to why the damage to the column wasn’t spotted during an earlier round of inspections of the T tunnels following the March collapse of a section of the garage, which sent 100 tons of rubble and steel onto the ground above the tunnels.

Those earlier inspections in March of the subway tunnels underneath the garage did not look at the support columns for the Government Center garage, a spokesperson for the MBTA said in an email. 

While the garage’s support columns passing through the MBTA’s tunnels, “they do not support the tunnel structure in any way and were not related to the March incident or part of the T’s investigation and inspection,” the spokesperson stated.

Rather, that was HYM’s job to this, with the developer having inspected the columns when it first began redeveloping the Government Center garage site years ago, the T contends.

In addition, HYM’s engineers, who accompanied the T’s engineering team during the March inspections, “had the opportunity at that time to also inspect these columns,” the spokesperson wrote.

While HYM is effectively arguing its demolition work was not to blame for the damage to the support column, the T continues to take a different view, standing by its initial statement on June 23 that ties work on the Government Center garage redevelopment to the shutdown of the tunnels underneath.

"This service disruption as a result of HYM's project is unacceptable and the MBTA will seek to hold HYM Construction accountable for all costs associated with this event," MBTA General Manager Steve Poftak said in a statement at the time.