Robert Stewart Flores, a divorced father of two and Gulf War veteran, then used one of his five handguns to kill himself, reports CBS News Correspondent Sandra Hughes.
"We were all just in complete in shock," student Gena Johnson told CBS News Early Show anchor Harry Smith. "We were under our desks and we didn't know what to do. There wasn't a peep in the room.
"He excused two students by name and then we figured, OK, well, he's excused them so the rest of us are not going to make it."
About 100 people gathered for a candlelight vigil at the university Monday night.
Students who knew him say Flores was doing poorly in school.
"I know he was very stressed in a lot of different things and had a difficult time adjusting," Kimberly Ammons told CBS News.
"He came across as very aggressive and mean and seemed to have a lot of issues with being angry," said Lori Schenkel, a fellow nursing student.
In fact, an instructor told police the student had thought about "ending it all" and "might put something under the college." a year-and-a-half ago.
Instructor Melissa M. Goldsmith told police that Flores said he was having problems with a paper but also had a lot of problems other than school, according to the university police department report filed on April 24, 2001.
"He was depressed and thought about 'ending it all.' Flores then stated he 'might put something under the college,"' according to the report, which was provided to The Associated Press on Tuesday by university police.
The report said an officer called Flores and left a message. "I will follow up at a later date and contact Flores," the report said. It was not clear whether police followed up.
Sharon Ewing, a clinical professor at the College of Nursing, said it was common knowledge among the faculty that Flores was depressed. She said all three victims had tried to help him.
"Mr. Flores ... conveyed to staff that he was depressed, that he was thinking of ending it all, and that he might take some actions against the College of Medicine," said university police chief Anthony Daykin.
Besides the handguns, the 41-year-old Flores had at least 200 rounds of ammunition with him.
"Mr. Flores went in there to create a holy hell for our community," Tucson police Chief Richard Miranda said. "I can't imagine what the building would look like if he didn't have a sudden change of mind."
Bomb squad members were called in after a backpack or package was found underneath the gunman's body. The suspect had threatened to blow up the building, though it was unclear when the threat was made, police said. The college and nearby buildings were evacuated but no explosives were found.
Police said Flores first killed professor Robin Rogers, 50, in her office on the second floor of the nursing school building. He then went to the fourth floor, where he entered a classroom full of students taking a test being given by two teachers.
There he confronted Cheryl McGaffic, a 44-year-old ethics teacher who studied the relationship between health and spiritually in seriously ill patients.
He told McGaffic "he was going to give her a lesson in spirituality," said student Laura Kelley. Witnesses said Flores fired two shots into McGaffic's chest and stood directly over her as he shot her in the head.
Barbara Monroe, 45, was allegedly Flores' last target. The instructor was cowering behind a desk as Flores approached, witnesses said. "He asked her if she was ready to meet her maker. She said 'Yes,' and then he shot her once and then twice more," said Johnson.
After shooting her, he told the other students to leave. He was later found dead by officers searching the school.
Flores worked at the Veterans Affairs hospital in Tucson as a licensed practical nurse and was studying to become a registered nurse. He was employed by a nursing agency, said Spencer Ralston, associate director for the Southern Arizona Veterans Administration Health Care System.
Flores failed a pediatric nursing class and was struggling in a critical care class this term, said University Vice Provost Elizabeth Irvin. A critical care exam was being administered when the gunman burst into McGaffic and Monroe's classroom.
Fellow student Lori Schenkel said Flores bragged to pediatrics classmates last year that he had received a concealed weapons permit. She said he seemed to enjoy calling attention to himself by asking inappropriate questions and challenging instructors. He failed that class and had to take it again, Schenkel said.
"Most of the people in class didn't like him," Schenkel said. "He was very obnoxious and rude."
William Gordon, a registered nurse who said he worked with Flores at the veterans hospital and knew him for three years, said he saw nothing that would foreshadow violence by Flores.
"He was very nice, very intelligent, very well-spoken," Gordon said. "I never heard anything violent period from this guy."
"He was really, truly a nice guy," said Ammons. "I've never seen him be not gentle.
"Having some stranger come in and do it is one thing, but somebody you went to school with for three semesters ... it's shock. You just can't believe it," she added.
School officials said there was no follow-up, but that this was an "isolated incident" that campus security couldn't have prevented. However, they say they will conduct an assessment of campus security.