Riders are once again able to traverse downtown Boston on the MBTA’s Green and Orange lines after the agency’s engineers on Sunday evening declared tunnels safe for subway travel, capping off days of upheaval for the already-troubled agency.

An MBTA spokesperson on Sunday said the HYM Investment Group, which is demolishing the Government Center Garage for its Bulfinch Crossing mega-project, “installed the necessary supports to uphold the structure.” A team of MBTA and third-party engineers and safety experts inspected infrastructure that passes through T tunnels and assessed safety, then deemed it sufficient to resume service, the T said.

While the parties worked together over the weekend to address the problem, they have not presented a clear, united front on how to carve up the blame for another disruption in the area following a partial collapse of the garage in March that also led to days of subway closures.

On Thursday, the MBTA abruptly halted subway service on downtown segments of the Green and Orange lines, blaming HYM for the “unacceptable” disruption and calling it a “result” of the company’s garage project. HYM said in response that the issue was caused by “years of water damage” that was “unrelated to the demolition work.”

The MBTA continued to insist Monday that even if HYM attributes the deterioration to water damage and not directly to demolition work, the infrastructure responsible for the subway headache remains the company’s responsibility.

Decades ago, the T awarded an easement to the Boston Redevelopment Authority and its successors allowing the installation of columns and foundations to support the garage. The columns do not support subway tunnels themselves “in any way,” MBTA spokesperson Lisa Battiston said.

“HYM engineers inspected the columns as part of their initial project and plans as the columns are their responsibility,” Battiston said. “Regardless, HYM engineers were also present during the inspections in March following the garage collapse and had the opportunity at that time to also inspect these columns. Again, they are the responsible party for maintaining and repairing all columns and foundation footings that pass through MBTA tunnels to support the private Government Center Garage.”

An HYM spokesperson did not immediately answer News Service questions on Monday, but Gov. Charlie Baker took an opportunity to weigh in during a Monday press conference.

“That beam belonged to the building dating back to the ’60s when the building was originally constructed,” Baker said. “HYM had two years to inspect and make decisions about how they were going to actually do the process associated with taking down that garage. I think the T did yeoman work working with HYM over the course of the weekend to make sure that beam — which belongs to the building and belongs to HYM — was shorn up and secured so there wouldn’t be issues associated with the morning commute.”

On Friday evening, hours after offering an account of events that conflicted with the T’s, the company said in a statement that it is “not in the business of pointing fingers.”

“We are looking to solve a problem that affects the people who live, work and commute in this City,” HYM and John Moriarty and Assoc. said. “Yesterday, as previously stated, HYM’s team of engineers under the supervision of the MBTA, confirmed a problematic subsurface column within the MBTA tunnel. Upon detection, our teams immediately notified additional members of MBTA leadership of the issues the condition of this column posed. These tunnels are inaccessible without the permission of the MBTA.”

“We are thankful that we discovered this issue when we did and have been working closely with MBTA staff to rectify this issue, allow for the reopening of Congress Street and the resumption of MBTA service,” the company continued.

MBTA General Manager Steve Poftak said last week that the transit agency would push for HYM to cover all costs associated with the disruption. Battiston told the News Service the T is “in the process of determining the financial impact and will hold HYM fully accountable.”

The MBTA is under a microscope in the wake of a safety incidents including derailments and occasionally deadly malfunctions.

Federal overseers ordered the agency to make immediate changes after the early stages of an investigation found major risks posed by staffing shortages, insufficient protections against runaway trains, lapsed certifications and a backlog of delayed maintenance.

A lack of available dispatchers prompted the MBTA to slash weekday subway service on the Red, Orange and Blue Lines starting one week ago.

Lawmakers plan to get in the mix, too, and convene their own oversight hearing to probe failures and needs at the T. The House has also approved $400 million to help the transit agency comply with federal mandates.