Jennifer Rosen was scheduled to depart Kennedy International Airport on an 8:55 p.m. flight to San Francisco but instead found herself stuck at Gate C62 in Terminal 2. It was the typical misery of summer airline travel — or so it seemed.

A short distance way away, in another terminal, something more unpredictable than the weather had begun to sweep across the New York airport:

Panic.

It spread quickly and without warning.

By the time Sunday night was over, Ms. Rosen, 32, had sprawled out under a table seeking cover, followed a crowd of people who bolted through a secure door onto the tarmac, frantically called her sister to find out what was happening and tell her that she was alive, and, finally, made a mad dash from the terminal to join mobs of travelers who thought they might be living through an episode of terror.

In the end, it proved to be a false alarm.

While the authorities were still trying to piece together exactly how a report of gunfire at 9:34 p.m. outside the security checkpoint at Terminal 8 led to complete turmoil across one of the nation’s busiest airports, the accounts of passengers in interviews and on social media offered a lesson in the anatomy of fear.

It was a night of confusion and dread, informed by the latest headlines — including reports of recent attacks on airports in Brussels and Istanbul — as much as fact. In the absence of official information or instructions, unconfirmed reports from social media fueled the hysteria.