The unidentified man may have been lost, confused or simply mistaken about which way to go when he bypassed security and walked in through an exit door at Newark Liberty International Airport.
Whatever the explanation, his seemingly innocuous actions Sunday evening wreaked havoc on airline schedules and delayed passengers around the globe for the next 24 hours _ and again demonstrated that for all the new high-tech passenger and baggage screenings that have made air travel safer, human error can quickly send the whole system into a tailspin.
Transportation Security Administration officials on Monday were still investigating the incident, which occurred in the airport's Terminal C at about 5:30 p.m. Sunday.
An unidentified security officer assigned to the area of the exit door apparently didn't see the man enter but was notified by a bystander waiting for relatives to arrive. The security officer was reassigned to non-screening duties pending the completion of the investigation.
"Our protocol is to have a security officer stationed at the exit lane, and their function is to allow arriving passengers out and to make sure bystanders who have not been screened on the public side do not enter," TSA spokeswoman Ann Davis said. "TSA will look at all the circumstances and make a decision as to what further action is required."
Flights were grounded for about six hours while authorities looked at surveillance tapes to try to identify the man. He was later seen on tape leaving the terminal about 20 minutes after he entered it.
The timing of the security breach preceded by mere hours the implementation of stricter security measures at foreign airports in the wake of an alleged attempt by a Nigerian man to set off an explosive device on a Detroit-bound jet on Christmas.
That created a scenario in which international travelers described being frisked Monday as they prepared to board flights in Hamburg and Stockholm, then arrived at Newark where thousands of passengers had been stranded, some overnight, by what could turn out to be a split-second blunder.
Other airports have been affected by similar incidents in recent years:
_In July 2007, flights at Oakland International Airport were delayed by as much as two hours after a man walked into a secure area without going through security screening; six months earlier, another man ran past a security checkpoint and eluded authorities, causing another two-hour delay.
_Passengers on six departing flights had to be re-screened at California's John Wayne Airport in July 2006 after a woman bypassed a security checkpoint.
_In August 2005, passengers avoided security screening by walking through unlocked doors at Atlanta's Hartsfield-Jackson International Airport twice in two weeks. In one instance, two men were redirected and sent through security screening, but a surveillance video showed two other men had gone the same way; they were not found.
_Nearly 2,000 passengers were evacuated from Mineta San Jose International Airport in December 2002 after a man wandered past security screeners and into a boarding area.
One way to avoid these incidents is to lessen reliance on monitoring by human eyes, possibly by locking exit doors from the outside, said Martin Pollner, a former director of law enforcement for the Treasury Department who helped establish a program in the 1970s that put federal marshals on commercial airliners.
"They should think about changing the system so you don't leave it to a person sitting there seeing who's going down an exit," Pollner said. "You control it so there's no access."
Davis defended the TSA's actions at Newark and said the only way to determine whether the man had left anything in the terminal or passed something to another person was to "rescreen everyone and conduct a full sweep of the terminal."
She didn't speculate on whether the security officer had been distracted or was temporarily away from his or her post, but said the exit lane is well-marked.
"There are signs on both sides and some stanchions, so it's pretty clear to people it's not appropriate to walk down the lane in that direction," she said.
Associated Press news researcher Jennifer Farrar in New York contributed to this report.