The War on Terror has a new front line — the school bus line.

Financed by the Homeland Security Department, school bus drivers are being trained to watch for potential terrorists, people who may be casing their routes or plotting to blow up their buses.

Designers of the School Bus Watch program want to turn 600,000 bus drivers into an army of observers, like a counterterrorism watch on wheels. Already mindful of motorists with road rage and kids with weapons, bus drivers are now being warned of far more grisly scenarios.

Like this one: terrorists monitor a punctual driver for weeks, then hijack a bus and load the friendly yellow vehicle with enough explosives to take down a building.

An alert school bus driver could foil that plan, security expert Jeffrey Beatty recently told a class of 250 of drivers in Norfolk, Va. After all, bus drivers cover millions of miles of roads. They know the towns, the kids, the parents.

"The terrorist is not going to be able to do some of their casing and rehearsal activity without being detected by one of you," said Beatty, an anti-terrorism veteran of the CIA, FBI and the Army's Delta Force. The more people watching, he told the drivers, the safer the community will be.

With bus drivers becoming informal intelligence gatherers, the reach of homeland security is growing — not exactly what parents think of when their kids head to the bus stop.

The program demands strong oversight, said John Rollins, a former senior Homeland Security intelligence official now with Congressional Research Service.

Otherwise, he said, some bus drivers could think of themselves as undercover agents.

"Today it's bus drivers, tomorrow it could be postal officials, and the next day, it could be, 'Why don't we have this program in place for the people who deliver the newspaper to the door?"' Rollins said. "We could quickly get into a society where we're all spying on each other. It may be well intentioned, but there is a concern of going a bit too far."

Most school bus drivers do the job part-time, often to supplement other income. Many are retirees, mothers with young children, people between jobs, or school employees who also work as mechanics, janitors or classroom aides, according to government and industry officials.

The drivers are not being trained to be police. Their role is to report suspicious behavior to dispatchers, who alert the police and funnel tips to a national analysis center.

The new effort is part of Highway Watch, an industry safety program run by the American Trucking Associations and financed since 2003 with $50 million in homeland security money.

Leaders of Highway Watch worked with the school bus industry to tailor training for drivers, who are trusted each day to ferry 25 million children to and from school.

So far, tens of thousands of bus operators have been trained in places large and small, from Dallas and New York City to Kure Beach, N.C., Hopewell, Va., and Mt. Pleasant, Texas.

"As a bus driver, going down the same streets and going into the same neighborhoods every day, you know when there's a car that shouldn't be there," said Bob Pearson, who drives a school bus in Fairfax County, Va. "You have to realize that a school bus goes everywhere."

When he worked as a homicide detective, Pearson gathered tips from everyone on the roads — truck drivers, trash men, mail workers. So to him, recruiting bus drivers is logical.

Down in Norfolk, Shelita Hill, a driver for 23 years, acknowledged that she never thought of her school bus as a target of terrorism until she heard Beatty speak. Neither had many others in the class.

"He woke us up," Hill said.

Schools are the kind of target that terrorists want, Beatty said: a place where an attack could have huge symbolic impact and lead to mass casualties and spectacular images.

To underscore the point, he reminded drivers of Beslan, Russia, where terrorists stormed a school in 2004, killing 331 adults and children in a storm of gunfire and explosions.

In Virginia, bus drivers were taught how to identify and evaluate unusual activity. What drew your attention to this person in the first place? Is someone unfamiliar taking photos or drawing sketches of the area? Is the person asking a lot of questions about the bus route?

Then the drivers got tips on how to report what they saw: Jot down facts immediately. Back away from the situation to get a broader view. Are there accomplices?

Next came the security sweep. Drivers were shown how to inspect their buses, not just for routine maintenance flaws, but also for tampering by terrorists. A bus has lots of hiding places for a bomb — the glove box, luggage bins, the engine compartment, the first-aid kit.

Victor Manuele, a longtime school bus driver in New York and now in Norfolk, said he has been doing pre-trip safety inspections for years. Just not for explosives.

"I don't think I ever thought about, 'Oh, well, here, let me check my bus for a bomb,"' Manuele said after the training. "So, you know, all of that stuff is very helpful."

Kenneth Trump, a school safety consultant who tracks security trends, said being prepared is not being alarmist. "Denying and downplaying schools and school buses as potential terror targets here in the U.S.," Trump said, "would be foolish."

When drivers finish their training, they get confidential School Bus Watch ID numbers. They are reminded never to profile people as suspicious based on culture or ethnicity.

"They know what looks right and what looks wrong," Beatty said. "All we can do is ask them to use their judgment."