LOS ANGELES (Reuters) - A security panic that crippled Los Angeles International Airport and sent hundreds of passengers fleeing from terminals was triggered by reports of gunfire that proved false but were amplified by word-of-mouth and social media, police said on Monday.

The shooting scare on Sunday night, which marked the worst security disruption in nearly three years at the second-busiest U.S. airport, began with police receiving a call reporting gunshots fired at a United Airlines gate inside Terminal 8, police said.

The initial call came in moments after airport police had responded to separate reports of a man wearing a black cape and mask and carrying a sword in the baggage area of an adjacent terminal, according to airport police spokesman Robert Pedregon.

The man, who had dressed as the swashbuckling western character “Zorro” to pick up an arriving friend, was questioned and released. Investigators believe his appearance was merely coincidental to the gunfire reports, Pedregon said.

As officers converged on Terminal 8, police began to receive additional reports of gunfire around the airport, some second-hand from passengers who had seen reports on Twitter and other social media, Pedregon said.

Hundreds of panicked travelers and employees bolted from five of the nine main LAX passenger terminals, some pouring through security gates onto the tarmac and others out of ticketing and baggage areas into the street.

“Arriving at LAX off flight when people started pouring out of term 4 onto Tarmac. Security said ‘shots fired. Run!’” NBC Nightly News television anchor Lester Holt said on Twitter. “Witness tells me she heard ‘pops’ in lax term 4. Others say they just heard ‘run!’”

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Motor vehicle traffic was halted at the horseshoe-shaped central terminal, and flights were halted on the south side of the airport as police searched for possible suspects or signs of gunfire, Pedregon said.

None was found. But the chaos had a cascading effect that took more than two hours to sort out, he said.

“It was a general panic that took place,” Pedregon said. “It was like a snowball, like dominoes, however you want to describe it.”

The source of the noises taken for gunfire remained a mystery the morning after the incident began at about 8:45 p.m. PDT (0345 GMT). “We are investigating it but we haven’t been able to confirm any source of those noises,” he said.

During the scare, 27 incoming flights were diverted to other airports, and airlines reported 281 flights - 120 arrivals and 161 departures - were delayed, LAX said. Airlines reported just two flights canceled.

Pedregon said the incident marked the worst security upheaval at the airport since November 2013, when a gunman walked into Terminal 3 with a semi-automatic rifle and opened fire, killing a U.S. Transportation Security Administration agent.

The scare came two months after police temporarily evacuated a terminal at New York City’s John F. Kennedy International Airport after reports of gunfire. It also turned out to be a false report.

A preliminary investigation of that incident, which also occurred on a Sunday evening, found no evidence of foul play or suspicious activity.

U.S. airport security officials have been on heightened alert in recent months after deadly attacks at international airports in Belgium and Turkey.