Authorities say three people were killed and the suspect died in a shootout with campus officers.
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Law enforcement sources tell ABC News the gunman is 67-year-old Anthony Polito, a college professor who previously worked in North Carolina and Georgia, and had sought a job at UNLV.
RELATED: Sources identify gunman in deadly UNLV shooting as 67-year-old professor; shooter killed by officers
Police say the gunman started shooting inside Beam Hall, home to the business school.
UNLV student Shainna Alipon said she heard the gunfire from inside the Student Union Building next door.
"We made our way down the stairs of the building just to escape and while we were walking down, we heard even more gunshots. I said oh my gosh, I hope the shooter doesn't come to the building where we're at," Alipon said.
Once Alipon was across the street, she took cell phone videos of students funneling out to safety.
Alipon is just three weeks away from graduating with a journalism degree.
"My editor-in-chief, he was kind of snapping me out of it, he was like, 'hey can I borrow your phone we have to take video of this, you know we're the press' and I was like, oh shoot - you're right. So I snapped out of it, started taking more videos, started talking to a little more people trying to see hey is everyone on your end okay? What do you think is going on? It kind of solidified for the first time today - oh I am a journalist this is like my job," Alipon said.
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Meanwhile, UNLV student and Bay Area native Kaitlyn Tinio was on the other side of campus when she got the alert from the school: "This is not a test. Run, hide, fight."
"So then I ran to the student apartments across the street because I know people who live here and I've just been here ever since," Tinio said. .
RELATED: Here's what we know about UNLV gunman Anthony Polito, who killed 3 at school
Campus will be closed for the remainder of the week.
"I don't feel safe anymore. Finals are next week and all of our finals are supposed to be in-person," Tinio said.
Tinio, from Hercules, described feeling relieved and overwhelmed.
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"It just makes me feel scared for the future because it's getting closer and closer to me, closer and closer to my family, people who I love," Tinio said.
The identities of the victims were not immediately released.
During a press conference, Sheriff Kevin McMahill said they are working on figuring out the suspect's motive.
"We will be conducting this investigation into the wee hours of the morning, there is no doubt. I know you have an insatiable appetite like I do to find out why, to understand motivation, and we will provide that to you at the appropriate time," McMahill said.
By nightfall, an organization brought therapy dogs to help ease the fear and stress of the day.