THE FLIGHT WATCHMEN

Chan shambled back, defeated, humiliated, to Alabama where he worked for the state police. It took 9/11, eight years later, to bring him back to the skies. He applied for a job as a federal air marshal. Flying under cover, he told fellow travelers he was in Òmortuary affairsÓ; they looked at him and believed it.

Last year, Chan was promoted to a supervisory position on the Watch Floor. Every day, when he crosses the lobby at work, a twisted steel girder salvaged from the 72nd floor of the World Trade Center's North Tower reminds him of the cost of another failure.

On this Tuesday morning, Chan finishes his run around the neighborhood, his shirt soaked, his breath short, his calves burning, his thoughts scrolling over incidents from the day before: a suspicious golf bag at the Savannah airport causes delays; a box cutter is found in Phoenix on a Southwest jet, wedged between seats 2F and 2E . . . As stressful as being an air traffic controller had been, this job is more so.

Chan stops at Kathy's front door and looks up at the sky. At any given moment, 6,000 planes are soaring overhead, crisscrossing America. As a boy, even one -- airborne -- seemed like a miracle. As a man in midlife, Chan wrestles with the dread of even one going down.

ÒThis is like a second life for me. I get a chance to make redemption for the mistakes I made,Ó he says. ÒI get a do-over, so I can tell Kathy's daughter I did it right.Ó

1125 Hours: Suspicious Selectees on Flight to Las Vegas (LAS). . . 1133 Hours: Disruptive Passenger on Flight Arrested at Philadelphia (PHL) . . . 1204 Hours: Firearm Detected During Checkpoint Screening at Birmingham (BHM) . . . 1225 Hours: Passenger Arrested After Behavior Detection Officer Referral at Minneapolis (MSP)

Chan takes a quick shower -- ÒI don't have any hair to washÓ -- while his BlackBerry vibrates on Kathy's television stand.

In Northern Virginia, meanwhile, at the Freedom Center, Chan's boss, Kent Jefferies, rises from his desk.

ÒExcuse me, Kent,Ó Bruce Brown says, poking his head through the door. ÒAnomalous radar target coming up the Potomac.Ó

Bruce is the division chief for the National Capital Region Coordination Center, which monitors the airspace over Washington. The NCRCC staff, including a liaison to the North American Aerospace Defense Command (NORAD) wearing an Air Force flight suit, occupies a pod on the Watch Floor. The men stare at radar feeds on three giant screens from the FAA, DOD and DHS. They scrutinize the green, glowing rings for incursions into the forbidden circle. Every month, there are 20 to 30.

Kent considers Bruce's report. A pair of Customs and Border Protection Black Hawk helicopters swept the area; the pilots found nothing. Still, supervisors at the White House are raising their vigilance to Òcondition yellow.Ó

ÒSometimes computers do weird things,Ó Kent says, spinning the wedding ring on his finger, as his thoughts go round.