Terrorist plot? Mile-high romance? FBI cracks airline security mystery.

In the moments before the F-16 fighter jets were summoned, the crew of Frontier Airlines Flight 623 feared that a passenger's frequent and unusually long trips to the aircraft lavatory on Sept. 11 were a sign that a potential terrorist plot was under way.

Later, when the plane had landed in Detroit, reports suggested that the peculiar bathroom breaks had been a reckless tryst between two passengers determined to “make out.”

In the cold light of Sept. 12, however, the Federal Bureau of Investigation revealed the rather mundane truth: It was simply a single man having an off-color day.

The story of Flight 623 from Denver to Detroit – as well as another Sunday flight in which airline crew requested fighter-jet escorts – was a different sort of reminder of 9/11 on its 10-year anniversary. As New York and Washington held memorial services, the two airline incidents marked a return, for one day at least, to the weeks when the fear of fresh terrorist attacks changed how Americans saw the everyday world around them.

There was some reason for the vigilance. On Thursday, the Department of Homeland Security announced that there was a "credible threat" for an attack over the Sept. 11 weekend. But reports suggested that threat centered around a truck bomb in New York or Washington.

The FBI said Monday that the three people detained from Flight 623 in Detroit did nothing wrong. In fact, the two men and one woman didn’t even know each other. One man was unwell, and the others in the row had to repeatedly stand up to let him pass on his way to the bathroom – which drew attention.

But never were any of the three in the bathroom at the same time, an FBI statement said.

A similar situation occurred on American Airlines Flight 34 from Los Angeles to New York. The flight crew became nervous when it saw people making repeated trips to the bathroom. They called for fighter-jet escorts, but air marshals sorted out the problem before the jets were needed.

After that flight, too, three people were questioned and released.

It turned out that "everything was copacetic," an FBI spokesman told AFP.