Off the nation's coasts, recreational boaters scan the waters for "suspicious" acts, from scuba diving in unlikely places to yachtmen sketching bridges or ports.
Manhattan's doormen learn how to spot packages that may contain biological weapons.
In Pennsylvania, amusement park operators train to recognize unusual phone calls or inappropriate requests for information.
Call them the new "first responders" in the war on terror. As average Americans, from truck drivers to handymen, are increasingly standing sentry, they're swelling the ranks of a citizens' army, always poised and on guard.
Last week, terror warnings sent law-enforcement officers fanning out across five financial buildings in Manhattan, Washington, and Newark. But grass-roots groups form another wall of defense, mobilizing in a nationwide watch for suspicious activity - from the supermarket to the state fair.
To some, it's the most effective, pervasive counterterrorism strategy there is. But even as officials warn that limousines or helicopters may be the next big targets, some worry that a sharp-eyed citizens' force could turn into an army of hypervigilant spies, one that may ultimately trample on civil rights.
"I have said for a long time that probably the last person who will have an opportunity to prevent an attack ... could be a 22-year-old deputy sheriff on a cold rainy night, someone who just sees something that's not right," says Randall Larsen, CEO and founder of Homeland Security Associates, a private consulting firm. "We need an alert, educated public ... but can't go too far. Where do you find that line? We don't seem to know."
For the most part, these citizen groups have voluntarily assumed their roles in the war on terror. In Pennsylvania, the state's Commission on Crime and Delinquency has been running antiterrorism training for everyone from Rotary Club members to small business owners.
Don Numer, the training supervisor there, says at least 60,000 residents have received training since 2002.
The program is a basic hourlong lesson to define terrorism, the best ways to report it, and suspicious signs - perhaps a phone call asking if a CEO is in or where he parks, Mr. Numer says.
The model will be taken national in September, says Eric Schultz, project director for USAonwatch, which provides terrorism-awareness training for Neighborhood Watch groups across the country.
Mr Schultz says 10,000 groups are registered with the National Sheriff's Association, and his organization hopes to reach out to even more communities.
Then there are Highway Watch, America's Waterway Watch, and Airport Watch. More than 10,000 truck drivers have joined Highway Watch, an American Trucking Association (ATA) initiative that trains drivers to notice and report emergency or suspicious situations on the road.
The ATA launched the program in 1998 and added an anti-terrorism component after 9/11. With a $19.3 million grant from the Transportation Security Administration (TSA), the ATA hopes to train 300,00 to 400,000 more drivers by December.
For driver Philip Gould, who works for Jevic Transportation of Delanco, N.J., the program was a chance for civic involvement. He joined Highway Watch in 2003. "I figured, I'm an American. I live in America. If we can't all pull together and do something for our own country what good are we?" Mr. Gould asks.
Experts say programs that train professionals to look out for specific risks can be effective tools. "These kinds of programs, particularly those that relate to critical infrastructure protection, are particularly useful," says Jack Riley, associate director of infrastructure, safety, and environment at the RAND Corporation. Truckers, for instance, are likely to notice if hazardous materials they transport have been tampered with.
And Mr. Larsen points to Israel, where he says an estimated two-thirds of suicide bombers are apprehended prior to detonating their bombs - which he attributes to an alert and educated public.
Mr. Riley is more skeptical of the average neighbor taking an active antiterror role. Though communities can provide response capability, they are unlikely to prevent attacks, he says. Before the terrorist attacks on 9/11, for example, some of the hijackers lived in American communities.
"I'm not aware of behavior they may have demonstrated in the neighborhood which would have led them to be caught," he says. "The neighborhoods themselves are not at risk of being attacked. Attacks will occur in public spaces."
Still, as threats of attacks on high-rises in several cities came to light, at least some doormen have begun training. In Manhattan, while the city braces for the upcoming Republican National Convention, the Service Employees International Union (SEIU) Local 32BJ is training members to recognize and respond to criminal and terrorist activities.
Since May 2004, 1,122 of New York's building employees - including doormen, janitors, and handymen - are undergoing training. In a four-hour session, they learn to identify threats and communicate effectively in emergencies, says Matthew Nerzig, a spokesman for 32BJ.
Those involved in training say their intent is not to make spies out of the citizenry. In Pennsylvania, for instance, Numer focuses on behavior, not appearance, to steer citizens away from racial profiling.
The Pennsylvania program features a slide show of individuals identified as terrorists, including Osama Bin Laden and Sarah Jane Olson, the "soccer mom" who pleaded guilty to possessing bombs with the intent to murder Los Angeles police officers.
The lesson: "It's not what someone looks like," Numer says.
Meanwhile, new programs continue to evolve. America's Waterway Watch, run by the US Coast Guard, is planning a national launch soon, says spokeswoman Jolie Shifflet. It will include brochures, stickers to put on boats, and video training to help "enlist the eyes and ears of the public." While many local units have implemented such training, the national program will help target the 70 million recreation boaters in the US.
The attention to average citizens is not necessarily a change in strategy, nor a signal that smaller attacks are likely. "As security gets tightened up at airports and other [places] terrorists might exploit, we naturally start moving down the list of things they might exploit next," Riley says. "So it makes sense that we're looking to sew up as many other risks as we can."
In Las Vegas, Gary Thompson, a spokesman for Harrah's Entertainment, says security and surveillance workers are getting new instructions to be on the lookout for potential terrorists.
"Employees' training has changed," he says. "The changes have been primarily with the security people on the [casino] floors. We've responded to increases or decreases in the homeland security levels." The changes aren't anything customers would notice, he says - but they're definitely there.
Even in areas where terrorism risks aren't the highest priority, neighborhood groups are flourishing. "There's been a groundswell of very interested citizens who want to make the homeland safe," says Mike Pacheco, the New Hampshire state coordinator for Citizen Corps, which is run by the Department of Homeland Security and gives citizens the opportunity to help protect their communities.
There are 20 Citizen Corps councils in the state, many of which were formed in the first quarter of this year, Mr. Pacheco says. There are 2,000 nationwide.
It was terrorism that prompted Rich Hanegan, a firefighter in Pelham, N.H., to form a local Citizen Corps Community Emergency Response Team (CERT) in 2003. But the group's focus has evolved to meet other community priorities, too, including flooding issues.
"A lot of people ask: 'What's going to happen in Pelham, New Hampshire?'" says Mr. Hanegan. But the need to be prepared - for natural disasters and terrorist attacks alike - is an opportunity that he says the community can't pass up.
It's that spirit, say many, that is fueling the expanding citizens' army. "I felt powerless after 9/11," says Numer. "These programs empower people. They give you a feeling, 'Hey, I can be a part of this.' "
• Alexandra MacCrae in New York and Cathy Scott in Las Vegas contributed to this report.