MIAMI — By the time Donald Robertson, a retired architect, arrived at Fort Lauderdale-Hollywood International Airport’s Terminal 1 on Friday afternoon, the mass shooting in Terminal 2, which left five people dead, had been over for at least an hour. The accused gunman, Esteban Santiago, was in police custody and the airport had largely returned to its normal rhythm — passengers were noshing, charging their phones, watching television. Things were so sedate that Mr. Robertson did not even realize there had been a shooting.

But as he made his way past the gate, Mr. Robertson looked up and saw a “stampede” of people, including what looked like law enforcement agents, racing toward him. “It was tremendous,” he said. “It was like a herd reacting. Everyone around me was diving under chairs.”

A rumor about a possible second gunman had spread across the airport, via social media, news sites and word of mouth. People panicked and the airport was shut down, a turn of events that upended what had seemed like an orderly finale to a tragic afternoon. What followed that single rumor was a rush of terror for travelers and airport workers as they fled to the tarmac, were separated from loved ones, injured themselves in the tumult, hid in kitchens and storage rooms and hunkered down behind luggage trucks.

Chaos and uncertainty unfolded over the next 12 hours.

Through it all, travelers said, they were frustrated every step of the way by the dearth of information about whether they were safe, where to go once the airport was shut down and how they could retrieve the thousands of items — 23,000 according to a tally by airport officials — that were scattered across terminals.