Amy Bishop pleaded guilty Tuesday in Alabama to fatally shooting three fellow college professors and injuring three others there in 2010, but Massachusetts authorities would not say how the plea might ­affect the case against her here in the 1986 death of her brother in their Braintree home.

District Attorney Robert L. Broussard of Madison County said in a phone interview that Bishop, 47, pleaded guilty in a Huntsville courtroom to one count of capital murder of two or more people and three counts of attempted murder. She had previously pleaded not guilty by reason of insanity.

Sentencing is scheduled for late September. Though Broussard had indicated last year that he would seek the death penalty, he said Tuesday that a plea agreement will be in play at the next court date. He would not discuss details of the agreement.

“I have no comment,” he said. “We have a gag order.”

The Associated Press reported that prosecutors agreed to recommend a sentence of life without parole for the capital murder charge and three life sentences for the attempted murder charges.

David Traub, a spokesman for the Norfolk district attorney’s office, declined Tuesday to discuss strategy for the Massachusetts murder case after Bishop’s plea in Alabama.

“We’ll be interested to see the sentence and will make our determination . . . in light of what occurs,” Traub said. “We’ll obviously be looking for additional information for us to make an assessment on our Norfolk County charges.”

Larry Tipton, Bishop’s public defender in the Braintree case, declined to comment.

Bishop pleaded guilty Tuesday to fatally shooting three colleagues during a faculty meeting at the University of ­Alabama in Huntsville, where she was a professor of biology, in February 2010.

Allegations emerged after the killings that a denial of tenure for Bishop may have played a role in the shootings. It was not immediately clear how her guilty plea would affect release of information on a possible motive for the slayings.

A lawyer for Bishop, Roy W. Miller of Huntsville, did not respond to requests for comment.

In the criminal case in Massachusetts, Bishop is charged with murder in ­Norfolk County in the fatal shooting of her brother Seth in 1986.

Though initially ruled an acci­dent, the shooting and investigation that followed came under intense scrutiny ­after the Alabama slayings. In April 2010, William R. Keating, then district attorney, launched a formal inquest into the 1986 shooting, which resulted in Bishop being indicted.

Bishop is incarcerated in ­Alabama while she awaits sentencing. Broussard said that under Alabama law, a defendant convicted of capital murder can only be sentenced to death or to life without the possibility of parole.

Authorities in Alabama could not be reached Tuesday to discuss the possibility of transferring Bishop to Massachusetts to face trial, if Norfolk prosecutors decide to go forward with their case. Traub also did not have information on a possible transfer.

Keating, now a US representative, said Tuesday that his primary thoughts were for the victims in the Massachusetts and Alabama cases.

He said that he sought the indictment in the 1986 case for reasons of justice and as a backup measure, in case Alabama prosecutors encountered difficulties.

“At the time, I did it in the event that something in Alabama may have taken a wrong turn in the case, to sort of preserve that option,” Keating said.

He added, “I thought someone had to stand for Seth ­Bishop in this process, and try to bring a sense of justice to that young man’s life.”

Bishop fled her home with the shotgun after the Braintree shooting, tried to commandeer a vehicle from an auto body shop, and aimed the gun at ­police officers before being taken into custody.

Transcripts of the 2010 ­inquest also show current and retired members of the Braintree police, State Police, and Norfolk district attorney’s office testified that the initial investigation into the 1986 shooting was deficient, echoing earlier comments made in interviews with the Globe.

Calls to numbers listed for Bishop’s husband, James E. ­Anderson Jr., in Huntsville, and her parents, Judith and Samuel Bishop in Ipswich, were not ­returned Tuesday.

A lawyer for the Bishops, Bryan J. Stevens of Quincy, ­declined to comment on Amy Bishop’s guilty plea, though he did say that her parents did not attend the court session.

In June 2010, Stevens released a statement on behalf of Bishop’s parents, in which they insisted that their son’s death was accidental.

“We know that what happened 24 years ago to our son, Seth, was an accident,’’ the statement said. “Despite all the finger-pointing among local ­police, State Police, and the district attorney’s office, there is no evidence that Seth’s death was not an ­accident.’’

During the inquest, Samuel Bishop testified that Amy had been traumatized the year ­before Seth’s killing by a burglary in the house, and he maintained that her fear contributed to events that day.

“And she made a terrible mistake in acting on that fear,’’ he said, according to the transcript.