Three people were killed and another seriously injured by a gunman who was shot dead by police at a US university yesterday.
US media have reported that the gunman was an academic who had applied for a professorship but was not hired.
The incident at the University of Nevada Las Vegas (UNLV), a short distance from the gambling hub's tourist-packed strip, was the latest mass shooting in the US.
Las Vegas Sheriff Kevin McMahill said there were three confirmed deaths and that the attacker was also shot dead by police.
A fourth victim had been critically injured in the shooting but their condition later was upgraded to stable.
US media reported the gunman was a 67-year-old career academic, with links to schools in Georgia and North Carolina, who had applied for a professorship at UNLV but was not hired.
ABC News, citing multiple law enforcement sources, identified the suspect as Anthony Polito and said the victims were faculty or staff, and not students.
US President Joe Biden in a statement decried the latest "horrific act of gun violence" to terrorise a college campus.
Police responded within minutes to reports of an active shooter at 11.45am local time (7.45pm Irish time), Sheriff McMahill told a press conference.
Two officers "immediately engaged the suspect in a shootout" and the suspect "was struck and is deceased at this time", said university police chief Adam Garcia.
The incident began as an outdoor gathering of students was taking place.
"Students were playing games and eating food, there were tables set up for them to build Legos," said Sheriff McMahill.
"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."
Police did not provide any further information on the identities of the victims or gunman, and are in the process of notifying next-of-kin.
One woman told local broadcaster KVVU that she had heard a series of loud noises and fled into a building on the campus, from which she was later evacuated by police.
"I was just having breakfast and then I heard three, like, loud booms," she told the station.
"Then two more, and then police showed up there and ran inside... but then after two minutes boom, boom, boom, more shots. So I ran into a basement, and then we were in the basement for 20 minutes."
Vincent Perez, a professor at the university, told MSNBC by phone that he had heard a lot of gunfire before taking cover on campus.
"I would say just seven, eight shots, one after another, loud and very loud," he said.
"As soon as we heard that, we ran back inside and we realised this is a real shooting, and there's an active shooter on campus," he added.
Officers were continuing to search the campus, which will remain closed through tomorrow, but "there is no more ongoing threat to our community," the sheriff said.
Las Vegas is a gambling and entertainment hub that attracts millions of visitors every year, many of whom come to see large, high-profile events.
Last month, the city played host to its inaugural Formula One Grand Prix. It is currently hosting basketball's NBA Cup at the T-Mobile arena, just 3km away from the scene of the shooting.
Los Angeles Lakers star LeBron James sent condolences to affected families during a press conference, in which he also voiced anger about the grim regularity of mass shootings.
"It makes no sense that we continue to lose innocent lives, on campuses, schools, at shopping markets and movie theaters and all type of stuff. It's ridiculous," he said.
Mass shootings are common in the US, a country where there are more guns than people, and where attempts to clamp down on their spread are always met with stiff resistance.
The country has recorded more than 600 mass shootings this year, according to the Gun Violence Archive, a nongovernmental organization that defines a mass shooting as four or more people wounded or killed.
The Washington Post, which keeps its own tally of mass shootings, said that as of Monday, there had been 38 such incidents in which at least four people had been killed.
Las Vegas was the scene of one of US's deadliest-ever mass shootings, when a gunman opened fire on a crowded music festival in 2017, killing 60 people.
Efforts to tighten gun controls have for years run up against opposition from Republicans, staunch defenders of what they interpret as an unfettered constitutional right to weaponry.
The political paralysis endures despite widespread outrage over recurrent shootings.
"This is not normal, and we can never let it become normal," said Mr Biden.
"For all the action we have taken since I've been president, the epidemic of gun violence we face demands that we do even more. But we cannot do more without congress," he said.
