THE FLIGHT WATCHMEN

The man is a selectee, a person flagged by the government as one who might pose Òa direct threat to U.S. civil aviation,Ó according to Greg Alter, a DHS spokesman. The selectee's boarding pass had been printed with a special mark. At the checkpoint, a guard was supposed to divert the man for additional screening. The guard missed the mark.

For Chan and the others who work the Watch Floor, the passenger at Chippewa in 4C triggers the first adrenaline uptick of the day. Chan is at home, about to wake Jamie, who is snuggled upstairs with her yellow blanky. Chan's shift doesn't begin until 2 p.m., but he is tracking alerts and incidents on his BlackBerry because in a few hours they will be his.

Every security breach across eight modes of transportation collects and dumps on the Watch Floor. The unmarked building, originally called the Transportation Security Operations Center, opened in August 2003. It responds to threats to mass transit, bridges, railways, vehicles and roads, pipelines, postal and cargo shipping, maritime matters and ports, and, above all, aviation. One minute, a report comes in about a mysterious truck abandoned on railroad tracks in Delaware. The next, a note is discovered on a ferry in North Carolina: There are bombs on this boat. Do not run. Only a warning. The next, a 78-year-old Egyptian woman in a wheelchair is trying to board a plane from Nashville to JFK with $9,800 in cash and eight boxes of razor blades in her bra.

ÒYou treat every incident, like --Ó says Chan's boss, Kent Jefferies, bracketing his eyes as if his hands were blinders on a horse, Ò-- is this the next 9/11? No? Good. Move on.Ó

One lesson of al-Qaeda's simultaneous strikes in 2001 is the importance of communication. Though run by DHS, the Watch Floor houses representatives from the Department of Defense, Department of Transportation, Secret Service, Capitol Police, FBI and FAA. Data from more than 450 federalized airports and 19,000 general aviation airfields feed into the Watch Floor. Analysts try to connect seemingly unrelated -- and unusual -- events as they unfold across the country, to spot trends, to stop an emerging attack. In the fall of 2005, for example, over the course of two weeks, passengers on three different flights stood up in the aisles and fainted. Was it a probe? Were terrorists testing emergency preparedness on airplanes, or were the serial faintings a coincidence?

ÒYou always hope it's going on somewhere, that the military is talking to the FAA, is talking to law enforcement, is talking to the airlines,Ó says Alter. ÒYou hope somehow, some way, those folks are talking, in case something goes bad.Ó

If things do go bad, or at least seem to, men in Air Force uniforms take action. ÒWe're scrambling now!Ó an officer said on a recent afternoon, running across the Watch Floor. Scrambling fighter jets. The classified radar had picked up an unidentified Cessna in restricted airspace over Washington. As Fox News reported the evacuation of the North Lawn at the White House and broadcast a live shot of people spilling out of the Capitol, a pair of F-16s roared toward the errant Cessna

ÒWe are not sentries. It's more activist than that,Ó says Kip Hawley, administrator of DHS's Transportation Security Administration, which manages the center. ÒOur job is not to sit and watch, but to stand and fight.Ó

Over his 13 months in the job, Chan has developed a sense about the threats, when they're routine blunders or world-class crazies, and when they might be real. At Chippewa, a flight attendant escorted the selectee out of seat 4C, for additional screening. The guard who missed the special selectee mark spent the rest of his shift in remedial training. TSA at Chippewa reported: ÒThere was no media attention.Ó And on this quiet Tuesday, Chan isn't alarmed. Not yet, anyway. His biggest concern at 8:30 a.m., is getting Kathy's daughter to the bus on time.

ÒHow's your day going to go?Ó he asks Jamie as they hurry along.

Chan is never sure. Kathy, a former travel agent who now makes reservations for air marshals, is the optimist. Chan has Òa doomsday outlook,Ó he says. ÒThere's an underlying, unknown anxiety and stress in those of us who deal with terrorist threats. We know we're getting farther away from 9/11 and closer and closer to the next attack. It's only a matter of time.Ó When Chan sends Kathy's 7-year-old girl out into the world, he worries.

At the bus stop, Jamie is the last kid in line. She steps up, and right before she disappears through the bus door, she always turns around. Chan looks at her: the sandy hair he has brushed and smoothed back with Jamie's favorite lime green headband, the freckles across her nose, the blue eyes. Chan waves. He calls it Òthat last reassurance wave.Ó