Why do Americans keep thinking their airports are under attack?
Frightened travellers fled Los Angeles International Airport (LAX) on Sunday in the latest case of panic at US airports and other public gathering places. Are Americans getting jumpier about the threat of attack?
Sam Macon, 36, had just picked up his luggage at LAX baggage claim when the screaming started.
"I couldn't tell who started yelling first," says Macon, a filmmaker who lives in LA. "But within minutes if not seconds I heard somebody shouting something about an active shooter."
Until then, the journey for Macon and his girlfriend back from Atlanta, where they'd been for a wedding, had been "uneventful". Then suddenly they were among hundreds of people - many screaming and pushing - rushing for safety.
The couple followed as "throngs of people" ran deeper into the building, away from the terminal exit. "The further people got away from the exit, the more hysterical they became," says Macon. "By the time we reached the departures area, some were panicking and crying."
Reports of a shooting sent passengers running onto tarmac and through security checkpoints without being screened, airport authorities said. Nearly 300 flights were delayed as a result of the security alert.
However, LA police said reports of a gunman turned out to be false. They are still investigating the source of "loud noises" that might have sparked concerns.
Minutes before the panic broke out, officers detained a man carrying a plastic sword and dressed as the fictional character Zorro - an event that Macon caught on camera. The man was questioned on suspicion of having fired a gun but was later released, police said.
The incident comes two weeks after parts of New York's JFK airport were closed amid reports of gunfire that later proved to be unfounded.
US media said applause from a group of passengers watching Usain Bolt's sprint in the Olympic 100m race started a series of events that snowballed into chaos.
In a separate case last week, nine people were injured when the sound of balloons popping and a fire alarm sent people running for fear of a gunman in a Florida shopping centre.
"What we are seeing is the power of the mind and the group when people are anxious or hyper-vigilant and concerned about threats," says Gary Small, professor of psychiatry and behavioural sciences at University of California, Los Angeles.
"And it's quite understandable what's happening - it's human nature. If you think you're in a dangerous situation you are going to respond."
The trigger for mass panic can be small and isolated, says Small. Although situations vary, the sound of a car exhaust pipe backfiring or comments by just one or two individuals can cause widespread alarm.
This is because people who feel anxious or face uncertainty will naturally look for explanations, Small says.
And heightened sensitivity about mass shootings and terror attacks in the US make the worst case scenario plausible.
"Gun violence has permeated our conversations and our existence," says Tricia Wachtendorf, director of the Disaster Research Center at the University of Delaware.
"If you are an everyday person and you hear there might be a shooter - and you have not got any information that contradicts that - it seems reasonable to me that you would move forward with an abundance of caution."
News organisations have been accused of fuelling fear with sensationalist headlines.
Meanwhile, Gary Small says terror attacks around the world - such as in France and Belgium - have contributed to an anxiety about the threats people face in their everyday lives, with airports and planes the subject of particular concern.
Certainly, Americans cannot easily forget about the dangers posed by groups such as Islamic State when these issues are at the centre of the presidential election campaigns.
And Small says the political atmosphere in the US is fuelling a general sense of anxiety that can affect people's reactions during highly pressured situations.
"People are quite frightened about the divisiveness in the political arena at the moment. We want to look to our politicians to calm us and lead the way, and without that people are feeling unsettled."
He says recent incidents suggest cases of panic and hysteria are becoming more common in the US.
However, Tricia Wachtendorf says people are actually less likely to panic than we are often led to believe. And while instances of people fleeing in alarm have led to injuries in the past, it's important to remember that these reactions can also avert greater disaster if a threat turns out to be credible.
"In these kinds of events, different people have different levels of anxiety," says Wachtendorf. "But widespread panic is extremely rare.
"We see actually people following orders, and helping each other. They are not engaging in anti-social or irrational behaviour. They are doing what they are being told to do."
Looking back now at Sunday's incident at LAX, Macon says the experience seems bizarre and even exciting. "Now that I know that everyone was okay and no-one was hurt, in retrospect that whole thing was quite thrilling," he says.
But he says he also learned a lot about how emergency events can unfold.
"I'm not going to be more paranoid now," he says. "But I will look at this as a practice run or training - in case something does happen in the future."