HUNTSVILLE, Alabama — A woman biology professor accused of murdering three University of Alabama colleagues after she had been denied a promotion was in US police custody Sunday, as a new report emerged about a fatal shooting in her past.

Amy Bishop, 45, a mother of four, was charged with capital murder and could face other charges including aggravated assault, district attorney Rob Broussard told a press conference in the US southern city of Huntsville.

Police said she used a nine-millimeter gun, armed with 16 bullets, at a university staff meeting on Friday. The gun was later found in the women's restroom.

A stunned Bishop, dressed in jeans and a pink sweater, was seen being driven away from the University of Alabama campus by police after the incident, shaking her head in disbelief.

"It didn't happen. There's no way. They're still alive," she murmured to local television station WHNT-TV as she climbed into the vehicle.

The shooting allegedly happened after Bishop, who had worked at the university since 2003, discovered several months ago she had been denied tenure, which would have secured her job in the biology faculty.

Meanwhile, The New York Times reported Sunday that Bishop fatally shot her brother in 1986 in suburban Boston.

According to police in Braintree, Massachusetts, 24 years ago, Bishop fatally wounded her brother, Seth Bishop, in an argument at their home, which police at the time called an accident, the paper reported.

However, Braintree police chief Paul Frazier said authorities were considering reopening the case because it may have been mishandled when Bishop was let go without being charged, the report said.

In Huntsville witnesses to the university shooting told local media screaming had broken out as the biology faculty met Friday in the math and science building, the Shelby Center.

The three slain faculty members were identified as Gopi Polia, the chair of the biology department; Maria Ragland Davis, a professor of biotechnology; and Adriel Johnson, a professor of physiology.

Two of the three people also injured in the incident were said to still be in critical condition on Saturday, while the third was in stable condition.

University president David Williams told AFP Saturday of his shock, saying his first reaction had been: "This can't be happening. It's incomprehensible."

The university, which has about 300 staff, has a "no-gun" policy on campus, he said. "We do not have metal detectors on our campus. This is a very safe community and it was a safe campus."

An e-mail alert sent to students Friday read: "There has been a shooting on campus. The shooter has been apprehended. Everyone is encouraged to go home, classes are cancelled tonight.... Counselors are available."

Williams confirmed about a dozen people had attended the biology faculty meeting, and said Bishop, whom he did not know well, had been informed several months ago that she would not be getting tenure.

The Huntsville Times said Bishop, a Harvard-educated geneticist and her husband, Jim Anderson, are credited with inventing a mobile cell incubation system, which could replace the old-fashioned petri dish.

The incident was just the latest in a series of school shootings to rock the United States -- most of which have been carried out by students -- amid the nation's ever-prevalent debate about gun control.

The shooting comes more than two years after the southern state of Virginia was stunned by the April 2007 massacre of 32 people at Virginia Tech University by a student gunman, Seung-Hui Cho, who then turned his gun on himself.

In 1999, two teenagers went on the rampage at Columbine high school in Colorado, gunning down 13 people before also killing themselves.

In the first six weeks of this year alone several shootings have already been reported around the country.

Last month, eight people were killed in Virginia by a lone gunman. And in early January a disgruntled employee at a Missouri plant of Swiss power company ABB went on a rampage shooting dead three people and wounding five others, before killing himself.