TUCSON, Arizona (CNN) -- A gunman believed to be a student shot to death two professors and a third person before apparently killing himself Monday at the University of Arizona College of Nursing, police and university officials said.
The building where the shootings occurred was swept for explosives afterward because of concern that the gunman might have had a bomb, authorities said.
Tucson Police Chief Richard Miranda would not say what, if anything, police found in the sweep. In a statement on the Internet, the university said two professors -- who witnesses said were women -- were shot. The fourth victim was found in the building during a search, police Sgt. Marco Borboa said. The victim was not identified.
Miranda said the gunman was among the victims, and the university Web site said he may have died from a self-inflicted wound.
Miranda said police learned Monday morning that the suspect had threatened to bomb the building in the last few weeks. He didn’t say from where the information came.
The chief also said a bomb-sniffing dog had a reaction to the suspect’s car, which was parked in a lot outside the college, and when police found a backpack under the body of the gunman, they suspected it might contain an explosive device.
Police are confident that the gunman acted alone, he added.
“I am personally comfortable that there is no further risk,” university President Peter Likins said. “It’s an intensely personal tragedy and trauma.”
A witness said a student angry over being barred from midterm exams entered a classroom where exams were being given and began shooting with a handgun. Police would not confirm that it was a student.
The witness said the gunman fired at one professor as a number of students ran out of the classroom, then fired at a second professor. The gunman, he said, then allowed the rest of the students in the room to leave.
A witness taking the test on the fourth floor of the building said she heard about 15 gunshots. It was “really scary,” she said. “It was silent while he was shooting our teachers. I was afraid that I was going to get shot.”
University spokeswoman Lisa Wakefield said that the college and surrounding buildings were evacuated after the shootings at 8:40 a.m. (10:40 a.m. EST). In addition to the College of Nursing, other buildings closed included the College of Pharmacy, Life Sciences North and Basic Sciences, police said.
The University Medical Center was locked down, and emergency cases were being diverted to other hospitals, a spokeswoman said. Visitors were barred, but she said employees were able to go in and out.
Police said people who fled the college after the shootings were gathering at a nearby alumni center, where counseling was available.