The biology professor charged with shooting dead three staff at an Alabama university on Friday is alleged to have also killed her brother 24 years ago, BBC reported.
Amy Bishop shot her 18-year-old brother in the chest, according to the current police chief of the Massachusetts town of Braintree where this occurred.
The case was logged as an accident and never came to court, Paul Frazier said.
Earlier, local media reported that Ms Bishop had opened fire during a faculty meeting after being denied a promotion.
Former colleagues have said the neurobiologist, who is in her 40s, was upset after losing out on a tenured position at the University of Alabama in Huntsville.
Police have refused to discuss a motive, and university spokesman Ray Garner has said the meeting was not called to discuss tenure.
The three people who were killed were the head of department and two other professors. Two of the three injured, also biology department members, are said to be in a critical condition in hospital.
Ms Bishop could face the death penalty if convicted of murder, officials said. Her husband is also reportedly being questioned.
The BBC's Madeleine Morris in Washington says the shooting has shocked many Americans, and the revelations about Ms Bishop's past in Massachusetts are sure to increase that feeling of unease.
At a news conference in her former hometown of Braintree, Police Chief Paul Frazier said the biology professor had killed her brother in 1986.
It was reported as being an accident at the time, but he said officers who were serving then had told him that Ms Bishop shot the young man after an argument and fled the scene before being arrested at gunpoint.
Mr Frazier alleged she was never charged after the then police chief, John Polio, or someone acting on his behalf, intervened and told officers to release Ms Bishop. He added that detailed records of the incident had been missing for more than 20 years.
"It is a far different story than what was reported at the time," Mr Frazier said.
But his account was disputed by Mr Polio, now 87, who insisted he had not told officers to release Ms Bishop and denied records had gone missing.
"Nobody was trying to cover anything up. We had a job to do. We did it, we used our own judgement under the circumstances," he said, adding that the district attorney had not complained at the time.
US educational establishments have seen numerous fatal shootings
The authorities in Massachusetts say they are talking to police in Alabama about the older case. No decision to reopen that investigation has yet been made.
US educational sites have seen numerous fatal shooting attacks in recent years.
The most serious occurred at Virginia Tech University in 2007, when a student killed 32 people before turning the gun on himself.