EGG HARBOR TOWNSHIP, N.J. -- Thousands of armed, undercover air marshals rushed into service since last year's terrorist attacks are flying carefully chosen missions, sometimes on an hour's notice because of a new terrorist threat.

Air marshals were aboard every flight in and out of New Orleans during the Super Bowl, patrolled the skies above Salt Lake City during the Olympics and routinely fly wherever President Bush is.

That wasn't the case on Sept. 11, when a handful of air marshals were flying incognito, all on the wrong planes, helpless to stop the terrorists.

"I know, I know we could have," Scott McShaffrey, a marshal who flew overseas that day. He shook his head grimly as he walked toward the marshals' command center here near Atlantic City International Airport. "I just wasn't in the right place at the right time."

Although many planes still fly without a marshal, the odds of having a team on the right plane have improved exponentially.

The program has mushroomed from the nearly forgotten 32-person unit of Sept. 11 to a sprawling federal law enforcement agency that employs thousands. The exact number is classified.

The program has its critics. Some say the marshals were too quick to draw their guns on a flight diverted to Philadelphia on Aug. 31 when passengers wouldn't sit down after an unruly man was arrested.

Growing pains

Thomas Quinn, a retired Secret Service agent who looks and sounds like Clint Eastwood, concedes there have been growing pains as thousands were hired, 21 field offices set up and consultants called in to create what he calls "one of the most sophisticated reservation systems in the world." Quinn took charge of the marshals program in February.

Most travel reservations are now made electronically by staff crammed into jury-rigged work stations inside trailers, an old garage, and metal buildings at the command center.

A giant flat high-definition TV screen inside the crowded Mission Operations Center recently displayed hundreds of missions being flown at that instant within the United States. There are about 25,000 flights per day within the country, and marshals also work overseas trips.

Larry Tormey, who supervises international operations, said fresh intelligence often forces them to switch flights.

"Sometimes we get threat information and have an hour to redeploy," he said.

Some airliners diverted by fighter jets because of an incident or suspicion had air marshals aboard who secretly told the military via the pilot that the problem wasn't serious, said Donald Anderson, assistant special agent in charge of Mission Operations.

Spending hours in an airplane looking for suspicious behavior is not a job for everyone. The average salary is about $52,000.

"There's always a bit of a personal sacrifice, going from city to city, living out of a suitcase," said McShaffrey, now a special agent in charge of administrative operations.

Have they prevented a hijacking?

"It's like protecting the president," said Quinn, who was second in command of President Reagan's Secret Service detail. "You never know who you've thwarted by your presence."