At least 59 people have been killed and more than 500 injured after at least one gunman opened fire on concert-goers on the popular Las Vegas Strip in the US state of Nevada.

The mass shooting took place late on Sunday outside a building at the Mandalay Bay Resort and Casino during the Route 91 country festival. More than 22,000 people were in attendance at the concert.

Police named the shooter as 64-year-old Stephen Paddock, who they said fired into the crowd from the 32nd floor of the Mandalay Bay hotel.

He was later found dead in his hotel room, a police statement said.

Eric Paddock, Stephen's brother, said on CNN that his brother had

"absolutely" no mental problems or political motives that the

family was aware of.

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Cole Watson, who was at the concert with his wife and kids, said: "At first it sounded like someone was setting off a whole lot of fire crackers".

He said when they "realised it was gunfire and everybody started running".

"The scene was insane - it was absolute chaos," Watson told Al Jazeera by telephone.

Wade Millward, a journalist with the Las Vegas Review-Journal, told Al Jazeera that the attack took place while country-music star Jason Aldean was performing.

"Witnesses told me they heard a sound like fireworks going off," he said.

"Aldean played on for about thirty seconds before it dawned on everyone that this was not fireworks. Then people hit the ground and people ran."

Watson said he saw several injured people when he and his family ran from the scene of the attack.

Aldean later wrote on social media that he and his band and crew were safe. He called the night "beyond horrific" and offered condolences.

Running all over the place

Another witness, Felipe Uribe, was watching the concert from the top floor of the Mandalay Bay hotel when he saw the attack take place.

"We were looking at the concert down below when we saw what appeared to be fireworks," Uribe told Al Jazeera. "It wasn't until later on that we realised it was actual gunshots."

He said that people were running all over the place and "they were laying on the floor on their backs or their stomachs".

"We didn’t know at that time the shots were coming from the hotel."

Police later took Uribe and others he was with to a secure location where they had to wait for the situation to be cleared.

Videos posted on social media show the crowd ducking for cover as multiple shots can be heard.

The Las Vegas Review-Journal's Millward told Al Jazeera the lobby of one of the hospitals where victims were taken was packed with friends and family.

"I've seen people wearing bloody clothing and there is discarded bloody clothing outside," he said.

"There are still people walking by with blood on their shirts. There is police everywhere and there are two helicopters flying overhead."

During a press conference, Clark County Sheriff Joseph Lombardo said that at least 59 people had been killed and more than 500 were injured, but added that the precise number of victims could not be given.

Two on-duty police officers were shot, one of them is still in critical condition, Lombardo said.

Lombardo added that police are still looking for a possible accomplice, who is wanted for questioning.

US President Donald Trump tweeted his "warmest condolences" to the victims of the "terrible Las Vegas shooting".

In a televised address Monday morning local time Trump called the attack "an act of pure evil".

Sunday's attack is the deadliest US mass shooting in modern history.

So far in 2017, the watchdog group Gun Violence Archive has documented 273 mass shootings in the US.

The group also recorded 11,621 gun-related deaths and 23,433 firearm-related injuries during that period.

The Mandalay Bay attack comes just weeks after Spencer Hight carried out a mass shooting during a gathering at his estranged wife's home in Plano, Texas.

The assailant killed eight people and was later shot dead by police.