The shooter was caught outside the Shelby Center, a science building, without incident, and no students were harmed, according to university spokesman Ray Garner. Local media reported the shooter was a faculty member, though Garner said he could not identify her. Officials said a man was also being detained.
Police said no charges had been filed and they were interviewing the woman suspect and a man identified as "a person of interest."
University spokesman Ray Garner said the three killed were Gopi K. Podila, the chairman of the Department of Biological Sciences, and two other faculty members, Maria Ragland Davis and Adriel Johnson.
Two others are in critical condition, and a third who was wounded was upgraded to fair condition. The injured were identified as department members Luis Cruz-Vera and Joseph Leahy and staffer Stephanie Monticello. Their specific conditions were not released.
Nick Lawton, the son of a biology professor at the school, said his father was not among the victims, but he did not know much more.
Lawton, 25, was exercising when a friend phoned him to tell him about the shooting. He called his father, Robert Lawton, and found out that he was not hurt, then he let rest of his family know.
"All I know is that my father is OK," Nick Lawton told The Associated Press.
Sophomore Erin Johnson told The Huntsville Times a biology faculty meeting was under way when she heard screams coming from a conference room.
University police secured the building and students were cleared from it. There was still a heavy police presence on campus Friday night, with police tape cordoning off the main entrance to the university.
The Huntsville campus has about 7,500 students in northern Alabama, not far from the Tennessee line. The university is known for its scientific and engineering programs and often works closely with NASA.
The space agency has a research center on the school's campus, where many scientists and engineers from NASA's Marshall Space Flight Center perform Earth and space science research and development.
The university posted a message on its Web site Friday afternoon telling students the campus was closed Friday night and all students were encouraged to go home. Counselors were available to speak with students.
It's the second shooting in a week on an area campus. Last Friday, a 14-year-old student was killed in a middle school hallway in nearby Madison, allegedly by a fellow student.
"This town is unaccustomed to shootings and multiple deaths," Garner said.
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Associated Press Writers Phillip Rawls and Desiree Hunter in Montgomery, Ala., and Jacob Jordan and Daniel Yee in Atlanta contributed to this report.