Reports of a gunman opening fire that turned out to be false caused panicked evacuations at LAX on Sunday night. Picture: Ringo H.W. Chiu

The security panic sent hundreds of employees and passengers fleeing from five of the nine main LAX terminals, some bolting through security gates onto the tarmac and others onto the surrounding streets.

Police have now confirmed the shooting scare, which took place about 9pm local time and led to major flight delays with at least three terminals in lockdown, was amplified by word-of-mouth and social media. It was the worst security disruption in nearly three years at the second-busiest airport in the US.

It’s also the second time in less than a month that a shooting scare caused mayhem at a major US airport. In both cases, the situation intensified via social media.

“People started to read the feeds that were going out over social media and retweeting and reproducing the messages without the specific details,” airport police spokesman Robert Pedregon told news.com.au.

“So we ended up getting information and reports of these incidents all over the airport and it caused a panic and hysteria that created such a big issue that it resulted in the majority of the airport being shut down.”

Mr Pedregon said airport security relied on all passengers to report any incidents they witness, but encourages them to be specific with the details.

“Particularly in this country and in this day and age, we need our citizens to feed us information and communicate with us,” he said. “We encourage all the time that if they see something to say something.”

The chaos began with police receiving a phone call reporting gunshots fired at a gate inside Terminal 8. Additional reports of gunfire then began to flood in, many from passengers having seen reports on social media, according to Mr Pedregon, who described it as “general panic”.

“The public got panicked, they were in fear of their safety and they evacuated through the fire doors. It was a dynamic situation and it’s unpredictable to know what citizens will do,” he said.

Some two hours later, police determined the reports of an active shooter to be incorrect, saying “loud noises” were to blame for the scare. Mr Pedregon described the caller as “a credible person” and said they are still yet to identify what the alarming sound was.

“Report of shooting at LAX proven to be LOUD NOISES only. No shots fired; no injuries. Investigation continues to locate source,” Andy Neiman, commanding officer of LA police media relations, confirmed in a tweet Sunday night.

While there were still delays, operations restarted Monday morning after the police action and a ground stop ordered by the Federal Aviation Administration caused airlines to divert 27 flights, including four international ones, to other airports.

Twelve of those flights were diverted to Ontario International Airport, with 281 incoming and outgoing flights delayed in total, and two cancellations. One person was injured from getting trampled, according to LAX.

The false alarm also caused traffic to be backed up leading to the airport for hours afterwards.

Passengers haul their luggage as they walk towards LAX after Sunday night’s shooting scare. Picture: Ringo H.W. ChiuSource:AP

Adding to Sunday night’s confusion at LAX was the bizarre arrest of a man wearing a black Zorro mask and costume, wielding a plastic sword.

The caped individual was captured on film at the airport as he was surrounded by at least six police officers with their weapons drawn and a police dog, in the time leading up to the evacuation.

The man was detained briefly until police were able to determine that the sword he had was plastic. He was later released, Assistant Chief David Maggard with the Airport Police Department told reporters.

The man told authorities he arrived at the airport to pick up a passenger and that he often works on Hollywood Boulevard dressed as Zorro, Chief Maggard added.

It remains unclear if the reports of an active shooter were related directly to the man in the costume.

The source of the noises taken for gunfire remained a mystery on Monday morning.

Sunday night’s shutdown at LAX closely mirrored a situation at New York’s John F. Kennedy International Airport two weeks ago when reports of shots fired inside Terminal 8 sent passengers and employees into a panic and shut down the airport. A boisterous celebration of the Olympics may have been misinterpreted as gunfire, authorities said afterwards.

Airports in US cities have been on heightened alert in recent months following deadly attacks at international airports in Belgium and Turkey.

LAX was the scene of a real shooting in November 2013 when a gunman walked into Terminal 3 with a semiautomatic rifle and opened fire, killing a US Transportation Security Administration officer. Alleged gunman Paul Anthony Ciancia wounded two other TSA agents and another person in the rampage. He was shot and wounded by police at the airport but survived.

Mr Pedregon confirmed Sunday’s incident marked the worst security upheaval at LAX since 2013.

Los Angeles International Airport was in chaos on Sunday night after a security scare.Source:Alamy