One additional gunshot victim was initially listed in critical condition at a local hospital and later upgraded to stable, police said.
Authorities said there is "no further threat" to the university, which has been closed for the rest of the day.
Authorities did not publicly release the name of the shooter. But multiple law enforcement sources told ABC News the deceased suspect is Anthony Polito, 67.
Polito had applied for a professor position at UNLV but was not hired, those sources told ABC. He had previously worked as a professor in North Carolina and Georgia.
Investigators were searching an apartment in Henderson, Nev. that was believed to be the shooter's home. Detectives have also obtained his phone and are searching for clues as to a motive for the attack.
Sources indicate the victims who were killed were faculty or staff members. It's still not clear if they were intentionally targeted but the fact that the victims were not students suggests to detectives it may not have been entirely random.
Police briefing on UNLV shooting
3 dead, gunman killed by officers after opening fire at UNLV campus
"Our hearts go out to the entire UNLV community," said Sheriff Kevin McMahill with the Las Vegas Metropolitan Police Department. "No student should have to fear pursuing their dreams on a college campus. What happened today is a heinous, unforgivable crime."
In addition to those struck by gunfire, the sheriff said four others were transported to local hospitals after suffering panic attacks and two officers incurred minor injuries during the search of campus.
McMahill declined to release the name of the suspect at a 5 p.m. briefing.
The motive remains under investigation.
UNLV President Keith E. Whitfield wrote in a message to the campus community:
"Today is a tragic day for UNLV. It is with the utmost sadness and grief that I am writing to you about a shooting that took place late this morning on our campus - our home. We're all still in shock as we process the unfathomable event. Members of our community lost their lives and others were injured. My heart aches for our UNLV family. I know all of us are sending our love and support to the victims, their families, and friends during this difficult time."
Timeline: What we know
The shooting was first reported on campus at 11:45 a.m. Shots were reported in the area of Beam Hall, home of UNLV's Lee Business School, as well as the school's student union, which is next door.
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Both campus police and Las Vegas metro officers responded to the scene.
Two detectives with the campus force ultimately were the first to locate and engage the suspect, according to campus police Chief Adam Garcia.
"Two of our detectives responded to the scene," Garcia said at a 5 p.m. briefing. "They immediately engaged the suspect in a shootout. The suspect was struck and is deceased at this time."
McMahill said the quick response likely prevented further deaths. A large group of students were gathered in the area for a campus event, where students were playing games, eating food and building Legos.
"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," the sheriff said at the 5 p.m. briefing. "Armed confrontation of the suspect by law enforcement stopped the suspect's further actions."
Students urged: RUN-HIDE-FIGHT
Shortly after the shots rang out, students were urged to hide or evacuate when possible.
"This is not a test," the university wrote in a post on X. "RUN-HIDE-FIGHT."
"Students hunkered down, they're afraid, they're scared," said Sheriff McMahill. "Our officers are going unit by unit, building by building."
Officers later escorted students off campus.
"Lots of cops on campus," one student said. "They had guns. We all walked out of the building with our hands up."
She said as she and her friends walked past the student union building, they could see a window shattered and pieces of glass scattered around.
Within about 40 minutes of the initial alert, Las Vegas police posted on X that a suspect "has been located and is deceased."
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A ground stop was initiated at Las Vegas' Harry Reid International Airport on Wednesday due to the close proximity of the airport runways to the shooting site, and because of the police response to the shooting, which included helicopters, according to an official briefed on the incident.
Classes and other activities were canceled, including a UNLV basketball game at the University of Dayton, Ohio.
The campus is expected to remain closed Thursday and Friday. Officials will determine if additional closures should extend into next week, when final exams for the semester are supposed to begin.
The shooting occurred in a city still scarred by one of the worst mass killings in U.S. history, the Oct. 1, 2017, mass shooting at the Mandalay Bay casino in Las Vegas, in which 60 were killed and hundreds more wounded. The UNLV campus is just over three miles from that location.
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UNLV has more than 30,000 undergraduate and graduate students at its 332-acre campus is less than 2 miles east of the Las Vegas Strip, the area's bustling tourist hub, and just across from Harry Reid International Airport.
The Associated Press and ABC News contributed to this report.