Footage has emerged of the moment elation turned to confusion and fear as patrons at the Pulse gay nightclub realised shots were ringing out on the dance floor.

Nursing student Amanda Alvear was one of hundreds of people who packed the popular gay nightclub, in the US city of Orlando, for its Latin theme night on Saturday.

The 25-year-old was a frequent user of messaging app Snapchat, and used it to post short clips from the club of her friends laughing, drinking and dancing. Her 'story' – a temporary compilation available for 24 hours – shows Alvear grinning as she holds up a full glass and leans close to her friend, upbeat music playing in the background.

As the night goes on, the friends start to dance, and she posts two more clips – of the group jumping and singing happily. Then, at 2am on Sunday, Omar Mateen walks into the club armed with two guns and starts shooting. An off-duty police officer starts to shoot back at him.

Alvear posts a final clip to her story. At just five seconds long, it is of her confused and worried face as gunshots echo in the background.

"Shooting," she says briefly, before staring into the distance as 10 gunshots rapidly ring out, followed by a pause, then another eight gunshots in quick succession.

The recording, which stops abruptly, would be the last time Alvear was seen alive.

Fifty people, including the shooter, died at the nightclub, making it the worst mass shooting in recent US history. Another 53 people were injured, some critically.

In the hours after the shooting, Alvear's family and friends launched a search for her on social media and stumbled across the video, which has been watched hundreds of thousands of times. Amanda's brother Brian Alvear confirmed the video was taken by his sister.

"Yes this is Amanda, she was creating a Snapchat story though the night this is the last time anyone saw her, after this she did receive and answer one call," he wrote.

It is believed she took the call while she hid in the bathroom.

On Monday, it was announced Alvear had died in the shooting. Her friend Mercedez Marisol Flores, 26, was also killed.

Friends of the pair began changing their Facebook profile pictures to images of the women, and Alvear's brother again took to Facebook to confirm she had "passed".

"I can't explain how loved my family feels right now in these trying times," he said. "Thanks for keeping me in high spirits and helping me be strong for my family."

In a media conference on Monday morning, local time, Orlando police chief John Mina said police reinforcements entered the club while the gunman was shooting.

Their gunfire "forced him to stop shooting and retreat to the bathroom", where he took hostages including Eddie Jamoldroy Justice, 30, who sent his mother frantic text messages saying he loved her.

In a bid to rescue the hostages, police then tried to blast a hole in the wall. When that didn't work, they then drove a BearCat armoured personnel carrier into it, making a hole large enough for the hostages to climb from.

"The explosive breach did not penetrate the wall completely," Orlando Police Chief John Mina said.

"There was a hole in the wall about two feet off the ground and about two or three feet wide. We were able to rescue dozens and dozens of people that came out of that hole.

"The suspect came out of that hole himself, armed with a handgun and a long gun, engaged in a gun battle with officers where he was ultimately killed."