Just after dawn on a recent May morning, Ajax, a 2-year-old black Labrador retriever, eagerly worked his way through a sparsely furnished room sniffing for explosives. On his third try, he picked up a scent behind a piece of furniture near the front of the room.

“Good dog, good dog,” said Andrew Baxter, his trainer, who reached into a pouch and threw Ajax a squeaky toy, much to the dog’s delight.

Ajax is one of 230 dogs at the Transportation Security Administration’s facility here on Lackland Air Force Base training to become bomb-sniffing canines. Dogs that pass the course will be deployed to the nation’s airports, the first line of defense against terrorist bomb attacks.

The assignment is becoming increasingly difficult as terrorists adopt techniques using household chemicals to construct bombs that make it hard even for a dog’s sensitive nose to discern.