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A man walks through Washington's Union Station with a backpack and puts it down by a potted plant. No one notices him.

He looks like any traveler. But inside his backpack are explosives.

Five minutes later, a yellow Labrador retriever named Zeta enters the same crowded room, straining at her handler's leash.

She has picked up the scent of black powder in the air and tracks it to the foot of the ficus tree, where she sits and waits eagerly for a reward.

This is not a thwarted terrorist strike but a demonstration of what a "vapor wake" dog can do.

For years, specially trained dogs have run their noses over objects to screen for explosives. But vapor wake dogs can detect explosives in the air despite crowds, cross-currents and other odors.

Their trainers say some of the dogs have been able to enter a room and pick up the scent as much as 15 minutes after explosives have passed through. Proponents say the dogs can detect a few grams of explosives.

Most are trained to recognize a dozen, but some dogs have learned as many as 20 types, including TATP, the explosive used in the London transit bombings, and ammonium nitrate, which Timothy McVeigh used to bring down the Alfred R. Murrah Building in Oklahoma City, Oklahoma.

If a dog needs to learn an explosive, it can do so in one or two days.

"Technology sometimes goes bad after two, three years, and you have to get new stuff," said Capt. William Parker of the Amtrak Police Canine Program. "A dog gets better as the years go on."

Parker says he is amazed by the dogs' capabilities and compares them to top athletes.

At an Auburn University kennel in Anniston, Alabama, yellow Labrador retriever puppies romp and roll in a grassy pen. They have been bred for vapor wake traits, but only 1 or 2 percent of them will make the cut.

Jeanne Brock, who manages the puppy program, says successful vapor wake dogs must have a strong instinct to hunt.

"They'll hunt and hunt and hunt and won't come back without it," Brock said.

John Pearce, who developed the program, says his best dogs are friendly but not overly social.

"They could care less about being petted," Pearce explained. "We are looking for a dog that is just very strong and very independent in searching, pick up air scent very easily without the direction of a handler."

At a few weeks old, the dogs are introduced to a variety of walking surfaces, like the slippery tiles they will find in a transit station. They are exposed to noises and large groups of people to make them more adaptable.

Older puppies are then placed in prisons in Mississippi, Georgia, and Alabama, where inmates begin teaching them to use their noses.

Eventually, they come back to Auburn and undergo a training regimen that is largely secret and is in the process of being patented. The dogs get 25 weeks of training, 14 of them with their permanent handlers.

"The dogs, when you get them, have more training than you do, and you have to play catch up," said Amtrak police officer Ed Ross, who works with Zeta.

The U.S. Capitol Police have just started using the $20,000 vapor wake dogs. The Federal Protective Service has a few. But Amtrak Police have made them a key part of their explosive detection strategy.

Parker says they provide a non-intrusive way of screening large numbers of passengers.

"You can take the dog to the situation; you don't need to take the situation to the dog," he said. "They can go anywhere and adjust to the situation. They can be out in the cold. They can be out in the wind or the rain or the storm. What equipment can do that?"

Although Amtrak's dogs have hit on fertilizer, nitroglycerin pills and photo paper, which all contain the same chemicals as explosives, Parker said all the situations were quickly resolved.

Rob Gillette, a veterinarian who is director of Auburn's Animal Health and Performance Program, says dogs' noses are remarkably sensitive instruments that sample the air several times a second.

He compares a vapor wake dog to a chef who can discern the scent of a particular herb in a complex stew.

For the dogs, Gillette says, finding explosives is simply a game:

"At some point, this one scent comes up, and he's like 'Oh, that's the one I like! That's the one I get rewarded for! I love this, so let's go find this!' "

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