On Thursday, four days before she departs for a new job as the new U.S. attorney for Massachusetts, Suffolk County District Attorney Rachael Rollins announced a criminal investigation into the MBTA in response to a crash on the Green Line last summer that injured 27 riders.

“There is perhaps no single state agency that impacts the daily lives of the millions of people who live and work in the greater Boston area more than the T,” said DA Rollins in a statement announcing the investigation on Thursday. “Therefore, it is imperative that if we see a continued lack of oversight or negligence at the MBTA that it is exposed and corrected.’’

Rollins also cited other serious incidents at or near MBTA facilities this summer, including a harrowing escalator failure at Back Bay station, a Red Line derailment, and the death of David Jones, a professor at Boston University, who died in a fall from a broken MassDOT-owned staircase adjacent to the MBTA’s JFK/UMass station in September.

“When their acts and omissions put the safety of community members and their own employees at risk, sometimes the only means of driving change is through the courts, and public demands for action,” said Rollins in her statement on Thursday.

Ironically, the investigation is getting underway just weeks after the MBTA finally launched a long-delayed project to improve train safety on the Green Line.

After two other serious crashes in 2008 and 2009 – one of them fatal – the National Transportation Safety Board (NTSB) recommended that the T install a “positive train control” system – technologies designed to prevent collisions between trains – on the Green Line.

But that recommendation languished for over a decade, as the T juggled other pressing priorities and a massive backlog in necessary repair work.

Two years ago, the MBTA’s Fiscal and Management Control Board authorized a contract for a new Green Line Train Protection System, a $170 million project. On December 15th, the T published its first monthly construction update for that project. It’s expected to be completed in 2024.