Originally Published: 07 DEC 23 03:00 ET

Updated: 07 DEC 23 08:10 ET

By John Miller, Elizabeth Wolfe, Taylor Romine and Evan Perez, CNN

(CNN) — The shooter who killed three people Wednesday at the University of Nevada, Las Vegas, was a 67-year-old career college professor, a law enforcement source said. And investigators are looking into the possibility that he may have been passed over for a job at the university.

The shooter, identified by law enforcement sources as Anthony Polito, died at the scene after a confrontation with police outside of a university building, according to Las Vegas Metro Sheriff Kevin McMahill.

CNN was told the investigative theory on a possible motive is that Polito sought a position at UNLV but was not hired.

He had worked for schools in Georgia and North Carolina, a law enforcement source told CNN. Polito’s LinkedIn page lists his most recent full-time employment as a business professor at East Carolina University, a position which ended in 2017.

The shooting began shortly before noon on the fourth floor of the university’s Beam Hall business school building, where students and professors were preparing for next week’s final exams, the sheriff said. Outside, students were gathered for an event with games, food and a Lego-building activity, he said.

The shooter moved through multiple floors before having an armed confrontation with law enforcement outside, which stopped the suspect, the sheriff said. It is unclear how the gunman died.

In addition to the three people killed by the shooter, one person was critically injured by gunfire, but that person’s condition later stabilized at a hospital, McMahill said. Four others were also taken to hospitals due to symptoms of panic attacks, he added.

Authorities have not released the names of the victims or disclosed whether they include students or staff members.

“We watched a lot of fear across the faces of those young men and women at UNLV today,” sheriff said.

The shooting prompted a campus-wide shelter-in-place order as authorities rushed to stop the gunman and then methodically worked to clear and evacuate Beam Hall and nearby buildings, McMahill said, noting they found groups of students huddled behind many of the doors.

Students anxiously took shelter in the student union across the street from Beam Hall, listening to gunshots and then waiting for police to evacuate them, a student who declined to be named told CNN affiliate KVVU. “A lot of people were panicking,” she said.

“We all walked out of the building, hands up,” the student said. “They evacuated us out of the student union. We walked past one of the windows, the window was shot through, glass everywhere.”

The fear and panic on campus evoked memories of the Route 91 Harvest music festival massacre that devastated the city in 2017, which remains the deadliest mass shooting in modern US history. Wednesday’s tragedy marks the 80th school shooting in the US so far this year, 29 of which have been on university and college campuses, according to a CNN analysis.

The university has canceled all classes through Sunday but is still considering how the campus will operate next week, when final exams are set to begin, UNLV president Keith Whitfield wrote in a Wednesday post on the school’s website.

“Today is a tragic day for UNLV,” Whitfield wrote. “We’re all still in shock as we process the unfathomable event.”

“I’m grieving for the victims of today’s senseless shooting, and my heart breaks for the many students, faculty, staff, parents, loved ones and community members who suffered through hours of painful uncertainty while officers ensured that our campus was safe and secure again,” the university president added.

President Joe Biden expressed condolences for the families impacted by Wednesday’s Las Vegas shooting and a killing spree in Texas this week. He also called on Republicans in Congress to work with Democrats to pass a ban on assault weapons and high-capacity magazines.

“Together, we must do more to prevent more families, and more communities like Austin, San Antonio, and Las Vegas, from being ripped apart by gun violence,” Biden said.

‘It looked like every single cop in Las Vegas was at UNLV’

Police began receiving reports at about 11:45 a.m. of a shooting at the university, prompting both on- and off-duty law enforcement to rush to the campus, the sheriff said.

Campus police also responded to the scene and engaged the suspect outside of Beam Hall, where students had been gathered for food and games just moments before, McMahill said.

“If it hadn’t been for the heroic actions of one of those police officers who responded, there could have been countless additional lives taken,” he said.

A student was sitting outside and eating breakfast when the shots began, the student told CNN affiliate KVVU.

“I heard three loud booms and I was like, ‘Oh, what was that?’” the student said. “Police showed up, then I ran inside.

“After two minutes, more shots. I ran into the basements, and I was there for 20,” the student said. “I was just hearing a lot of shots.”

Inside Beam Hall, a professor stopped mid-lecture after a loud noise rang out in the building, said student Brett Johnsen, who was in a second-floor classroom. But the sound didn’t initially seem like gunfire, so the professor resumed teaching.

“Then an alarm came on,” Johnsen said. “I’ve never heard an alarm like that before, it didn’t sound like a fire alarm.”

The students in the class began packing things up, relatively calmly, Johnsen said.

“When we began to walk out of the class, that’s when things got real,” he said.

The look on his professor’s face turned into panic and he urged the students to get back, lock the door and get on the ground.

“If the shooter came into our classroom we were all just basically sitting ducks,” Johnsen said.

A few moments later, the professor opened the door to check that it was clear and told the students to run. Johnsen said he ran as fast as he could down the stairs of the building and outside as far as he could go. Everyone on campus was urging each other to flee and evacuate, he said.

“It looked like every single cop in Las Vegas was at UNLV,” Johnsen added.

Shooting evokes memory of 2017 massacre

The university is just minutes away from the scene of the deadliest mass shooting in modern US history – the shooting at the Route 91 Harvest music festival on October 1, 2017, which left at least 58 people dead and hundreds more wounded. In the years after the massacre, two more victims died of shooting-related injuries.

Authorities investigating the shooting at UNLV referred to the 2017 massacre several times during a Wednesday news conference simply as “October 1.”

Sheriff McMahill said the training Las Vegas authorities have undergone since Route 91 contributed to how quickly they were able to respond and stop the shooter at UNLV.

“After 1 October and all the time and effort and energy that we’ve put in together – in training with the men and women of law enforcement, the fire service and EMS – watching how seamlessly they worked together today made me very, very proud to be their sheriff,” he said.

This is a developing story and will be updated.

CNN’s Cheri Mossburg, Sara Smart, Steve Almasy, Gillian Roberts, DJ Judd and Cindy Von Quednow contributed to this report.