SAN ANTONIO -- When O'Neill went to live with Amy Damon, he behaved like any other hyperactive 10-week-old puppy -- chomping on shoes, chewing up a rocking chair and tearing apart her children's stuffed animals. When Mrs. Damon tried to distract him with puppy toys, however, she didn't realize she was undermining homeland security.

O'Neill was in training to become a bomb-sniffing dog, and even something as harmless as too much play can wreck a career before it begins.

Bomb-sniffing dogs are becoming an increasingly important weapon in the nation's effort to protect its transportation systems. More than 370 bomb-sniffing dog teams now patrol 72 airports across the country -- an 86 percent increase since 2001 -- and more are on the way. This fall, the Transportation Security Administration began training dogs for mass-transit systems. The TSA dog program has a budget of $28 million this year, including $1.2 million for training.

The road from kennel to cop is a long one, marked by months of training and testing. The dogs -- mostly Labrador retrievers, German shepherds and Belgian Malinois -- have to go through the TSA's rigorous bomb-detection course at Lackland Air Force Base here. They must prove they aren't too blase or too excitable, that they have strong hips and focused noses. They face endless drills to sharpen their skills. Most important, they have to show their devotion to a simple thick rubber toy called a Kong, which they get to play with as a reward each time they find an explosive. "That toy," says Scott Thomas, chief breeder for the TSA, "has got to be the most important thing in their lives."

Many puppies don't make it to graduation. Most of TSA's bomb-detecting dogs are imported from Europe, especially Germany, where breeding bomb-sniffing dogs is an established, bustling business. And nearly half of them wash out of the training. Others are produced by the TSA's own breeding program, which began after the 2001 terrorist attacks to reduce the agency's reliance on imports. Of the 68 American dogs that have reached maturity, only 11 have been certified for bomb-detection work.

O'Neill, a black Lab, came from the TSA's breeding program. He and his five siblings -- Oliver, Oscar, Orio, Oakley and Otten -- were born on the base on June 9 last year. The program names each dog for someone killed on Sept. 11. O'Neill was named for John P. O'Neill, a retired Federal Bureau of Investigation agent who tracked al Qaeda for years before becoming the World Trade Center's security director.

When they were 10 weeks old, the puppies were sent to live temporarily with local families to get used to people and new environments. But soon after O'Neill's placement with Amy and Jeff Damon and their two sons, TSA officials began having qualms about how the family was handling the dog.

The journal the family kept mentioned frequent playtime. At a church event that promoted the TSA program, Mr. Thomas, who was present, says the Damons kept "throwing the Kong over and over until he was tired and didn't want to play anymore."

The concern: A dog bored with Kong doesn't have any incentive to hunt explosives. Mrs. Damon says that Mr. Thomas's recommendation for the destructive behavior at home -- putting O'Neill in his crate -- "seemed kind of cruel."

The European dogs don't have this problem. They spend a lot of their time as puppies confined, so play time is really a treat, says TSA trainer Jay MacClintock. The American dogs, by contrast, "are oversocialized," he says. The result: "A little piece of rubber doesn't do it for them."

In any case, TSA officials and the Damons agreed O'Neill should return to the base in March, three months early.

There, O'Neill spent most of his time in his kennel not doing much of anything. He was deprived of playtime, except for one or two 15-minute periods a week. Play, once again, was special for him -- a treat to motivate him to do the work for which he is being trained.

In the summer, after three months of play deprivation, O'Neill began training in explosives detection. Trainers hid the Kong and urged O'Neill to find it. Then they hid a sample of explosive (minus detonator) next to the Kong. The goal: to eventually remove the Kong and get the dog to track down the explosive simply by odor. It took O'Neill 40 to 50 trials per day over five days to learn the smell of the first explosive, and another five days to learn the second. After that, he learned one a day and eventually learned to recognize about a dozen different explosives, including dynamite, nitroglycerin and Semtex. Trainers taught him to sit and stare at the source of the odor when he found it. When he did, he got to play with his Kong.

The next big challenge came in October, when John Plouffe, a 30-year-old Massachusetts Bay Transit Authority official, arrived from Boston to begin training as O'Neill's partner. Mr. Plouffe admitted he knew little about bomb detection. "I'm playing catch-up with the dog," he said.

In their 10-week course, Mr. Plouffe and O'Neill ran through exercises in open fields, in a mock airport terminal and through an airplane fuselage. The pair performed well, but one problem emerged: O'Neill showed no interest in searching for bombs in lines of luggage. He darted away, pulling on his leash.

Trainers told Mr. Plouffe which bags had explosives so he could guide the dog. And they practiced indoors where there were fewer sights and smells to distract O'Neill. After three weeks of daily searches, O'Neill started to catch on. Mr. Plouffe also was learning a crucial lesson: how to tell when O'Neill smelled explosives. Every dog has a "tell," an early sign he has picked up the scent. O'Neill pricked up his ears and took deeper breaths while slowly wagging his tail.

Earlier this month, Mr. Plouffe and O'Neill completed their training but still faced a final challenge before they could start work in Boston: their certification test. "I'm a little nervous," Mr. Plouffe admitted as he waited to begin the luggage search, which entailed finding four bags containing explosives in a line of 30 bags. The dog sniffed the first bag but kept on going. At the second bag, he stopped, pricked up his ears and sat down, staring at the bag. Bingo. Out came the Kong. O'Neill found explosives in three other bags as well. "Good boy!" Mr. Plouffe said softly, as the two played together.

For their final test, man and dog had to methodically search a parking lot with 29 old cars, pickups and vans set up on jacks. When the pair reached the eighth car, O'Neill yanked his partner toward the ninth car, leading with his nose and crawling underneath the white pickup before sitting, staring and winning his Kong. After he discovered explosives in a few more cars, the two reached the end of the line.

"Are there any areas you'd like to go back to?" asked the TSA official conducting the test. Mr. Plouffe thought for a few moments. "I thought I felt something at No. 14," he said. After a minute or two, he added, "Let's go ahead and terminate" the search.

As it turned out, there were explosives in the 14th car, hidden in the driver's visor. Mr. MacClintock, the chief trainer, had noticed O'Neill's telltale change in behavior, as had Mr. Plouffe, and was dismayed that Mr. Plouffe didn't take another look. "I've told these guys a hundred times if you think something's there, follow your instincts," he said. Still, Mr. Plouffe and O'Neill passed the certification test because they had only missed one explosive; each team is allowed to miss two.

In a talk afterward, Mr. MacClintock stressed to Mr. Plouffe that there's no room for error in the real world. "It's not a drug dog, it's a bomb dog," he told him. "You don't want to find 99 percent. You want to find 100 percent." Mr. Plouffe nodded. By all accounts, he and O'Neill had done well, but he was annoyed at himself for the slipup. "I should have trusted the dog," he said.