SUFFOLK COUNTY DISTRICT ATTORNEY Rachael Rollins said Thursday that her office would open an investigation into whether the MBTA bore any criminal responsibility for the Green Line trolley crash last summer that injured 27 riders.

Rollins, who plans to leave the office on Monday when she is sworn in as the next US attorney for Massachusetts, said the investigation will focus on whether the transit agency failed to address the behavior of a trolley driver with a “reputation for speeding and history of violations.”

“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,” Rollins said in a statement announcing the probe. “Therefore, it is imperative that if we see a continued lack of oversight or negligence at the MBTA that it is exposed and corrected.”

The Green Line crash occurred last July when a trolley operated by Owen Turner slammed into another train from behind. National Transportation Safety Board investigators later determined that Turner’s train accelerated before the crash, and Turner was charged with negligence of a person having care of public conveyance and gross negligence in management of a train.

Rollins said Turner’s colleagues and supervisors had been aware of his pattern of reckless behavior but failed to take action.

“When an employer willfully turns a blind eye to one of their employee’s dangerous and reckless actions, they become complicit in behavior. When those employee’s actions rise to the level of criminal conduct, there are circumstances where an employer can and should also be held criminally responsible for the acts of an employee,” Rollins said.

MBTA spokesman Joe Pesaturo issued the following statement: “The MBTA has been cooperating with the District Attorney’s Office since the summer, and the T will continue to do so. Following September’s preliminary findings by the National Transportation Safety Board, the MBTA acted swiftly to address the conduct of the train operator, who was responsible for the collision. The MBTA will continue to emphasize safety in its employee training programs, and the T will hold any employees accountable for actions that adversely impact service or customer safety.”

Rollins also said her office had opened a perjury investigation into former Boston Police Detective John Brazil regarding his testimony in two murder trials.

Kevin Hayden, who is set to take over as interim Suffolk County district attorney.

With Rollins set to leave the district attorney’s office Monday, all ongoing investigations will fall into the lap of Kevin Hayden, the chair of the Sex Offender Registry Board, who was tapped by Gov. Charlie Baker on Thursday to serve as district attorney until a new prosecutor can be sworn in after the November elections.

The appointment marks a return to the office for Hayden, who spent about a decade as a prosecutor in Suffolk County after he graduated from Boston University’s School of Law, and it could offer him a foundation for a campaign should he choose to run for a full term.

Former Suffolk County District Attorney Daniel Conley, who oversaw Hayden for six years during his tenure, said he believes the SORB chair will make an “excellent leader” as the new top prosecutor.

“Like all good prosecutors, and I like to think I was the same way and I imparted this on the staff who worked for me, we recognize human frailty and we recognize that people who make mistakes are one class of individuals that come before the court. But there’s also another class of defendants who are very serious and violent,” Conley said in an interview.  “When Kevin was an assistant DA, he focused on both. He understood and he could distinguish the difference.”

Hayden, a Democrat, served in the Suffolk County DA’s office from January 1997 to April 2008.

Conley, who today works as an attorney at Mintz Levin in Boston, recalled that at the start of his tenure as DA in 2002, Hayden was a district court prosecutor and later rose through the ranks into leadership positions.

During his time in the DA’s office, Hayden served as chief of the Safe Neighborhood Initiative Unit and as assistant district attorney for the Anti-Gang Unit, the Homicide Response Team, the Juvenile Unit and for the Boston Municipal Court, according to Baker’s office.

Hayden departed in 2008 to join a private law practice, Conley said, and later began serving in general counsel and active executive director roles for the Sex Offender Registry Board.

In 2015, Hayden became the agency’s chair, replacing Gov. Deval Patrick appointee Anne Conners.

Conley said he anticipates Hayden will “cut his own path.”

“I expect Kevin to pursue a more traditional prosecutor’s role, one that is victim-centered and focused on serving victims and keeping neighborhoods as safe as they possibly can be,” Conley said. “That’s the kind of leader I expect that he will be because that’s the kind of prosecutor he was as a young lawyer.”

Voters will choose the next Suffolk County district attorney this fall. Hayden did not say on Thursday if he planned to launch a bid for a full term, but Conley said he hopes his acquaintance of two decades decides to do so.

“This is a great opportunity for the voters of Suffolk County to look at Kevin Hayden’s performance over the next eight months, for sure, until the Democratic primary and then 11 or so months until the final election,” Conley said. “My expectation is if Kevin does the job the way I expect he will based on my 20 years of knowing him, I think he should easily win election.”

Conley could serve as precedent, too: a Democrat, he was first appointed by Republican Acting Gov. Jane Swift to serve out the remainder of former DA Ralph Martin’s term before winning election later that year.

Biden in July nominated Rollins to serve as the next top federal prosecutor for the Bay State, and the Senate confirmed her in December via a party-line vote with Vice President Kamala Harris breaking a 50-50 tie.

Hayden’s appointment will also create a vacancy as chair of the Sex Offender Registry Board. Baker’s office said the administration is reviewing candidates to fill that role.