On Thursday, the T said there had been a construction vehicle derailment. Now it says there were three.

An MBTA employee looked over new Blue Line tracks with flashlight in hand heading toward the exit of a tunnel at the Airport Station.David L. Ryan/Globe Staff

The MBTA was asked the question over and over last week: How many times, exactly, had construction vehicles recently derailed during its Blue Line tunnel repair and upgrade project?

On Thursday, after a Globe inquiry, it said that “a construction tool cart derailed,” delaying the return of passenger service.

On the same day, the Federal Transit Administration said there had been two MBTA derailments in the past week, but didn’t say where. And then on Monday, the leader of the MBTA acknowledged there had actually been three Blue Line derailments, one on each day from May 7 to 9.

While General Manager Steve Poftak emphasized these were construction vehicles carrying tools, not passengers, that ran off the tracks, the inconsistent messaging is part of a pattern of opacity at an agency under review by federal inspectors.

“Just to clarify, there have been three derailments,” said Poftak, whose agency has for the last month been under intense scrutiny by inspectors from the Federal Transit Administration who are concerned with the T’s safety record.

“Derailments are never a good thing, but just to clarify, these were not Blue Line vehicles, these are not revenue vehicles, these were tool carts being used on the construction. We fully reported in full compliance with safety regulations that we work under. These were reported to the FTA and [Massachusetts Department of Public Utilities] in a timely manner. Obviously the site was frozen and a full investigation was taken each time,” Poftak said.

“I can assure the public that we will inspect every inch of that project before we resume service,” he added.

The project to replace track and improve flooding resilience in the Blue Line harbor tunnel — which connects the Airport and Aquarium stations — was supposed to be completed on May 8. After two delays, it is now expected to be completed Tuesday.

On Monday, T spokesman Joe Pesaturo said the derailments had happened on May 7, 8, and 9.

When the MBTA announced its first delay of the project on May 8, it did not mention that there had already been a derailment a day earlier. It did not mention the derailment that occurred on May 8, either. When it announced the second delay on May 12, the MBTA mentioned only one derailment. But by that point, there had already been three derailments.

The FTA, which is separately inspecting the T, did not immediately respond on Monday to questions about the three derailments. The DPU, which oversees safety at the T, has not responded to multiple requests for comment about the number of derailments reported by the MBTA.

Authorities at the MBTA, MBTA board of directors, Massachusetts Department of Transportation, and DPU found out about the inspection in a letter from the FTA on April 14, but did not acknowledge the probe publicly until the Globe obtained a copy of the letter last week.

The FTA said its inspection was spurred by a “pattern of safety incidents‚” including the April 10 dragging death of Robinson Lalin, whose arm got caught in a Red Line car at Broadway Station. Earlier this month, the NTSB reported that “a fault” in a door control system on the Red Line train contributed to the fatality. The MBTA is still waiting for delivery of hundreds of new Red Line cars from a Chinese company first contracted in 2014 to replace the more-than-50-year-old cars like the one involved in Lalin’s death.

The FTA’s safety management inspection is only the second time the federal agency has intervened on the local level in this way. In 2015, the agency conducted a safety management inspection of D.C.’s Washington Metropolitan Area Transit Authority that led to the federal takeover of safety oversight there for nearly three and a half years.

At a meeting of the MBTA board of directors safety subcommittee Monday, MBTA Chief Safety Officer Ron Ester said meetings with the FTA began last week and will continue for the next few weeks.

“We will cooperate with the FTA during this inspection,” he said. “When the FTA’s final report is issued, we’ll update the board on the contents.”

An FTA spokesperson said the agency expects a final report will be shared with the MBTA and then shared publicly by the FTA. An FTA spokesperson has said that the inspection will include the T’s subway and trolley systems but not the bus or commuter rail.

This is not the first time the MBTA has faced an outside investigation of its safety practices.

In 2019, the MBTA’s former board, the Fiscal and Management Control Board, assembled an outside group of experts to audit safety at the T after a series of derailments.

They found that the state agency lacked a culture of safety and provided 61 safety recommendations in six categories: financial review, safety assurance, safety culture, safety policy, safety promotion, and safety risk management.

As of February, two-thirds of the recommendations had been completed, including most of the safety culture and safety risk management recommendations, MBTA staff reported to the board, and one-third were in progress or on hold, including all of the financial review recommendations.

The panel found that the T was prioritizing delivery of its capital investments and paying “insufficient attention” to “day-to-day preventative maintenance and inspections and maintaining the full functionality of legacy assets.”

Governor Charlie Baker, who appointed most of the MBTA’s board members and the Secretary of Transportation, said last week he welcomes the FTA’s inspection.

Globe correspondent Grace Gilson contributed to this report.