Even in an era of high-tech crime-fighting, the best line of defense against a Brussels-style attack on airports and subways has four legs and a tail.

Dogs, with their exquisitely sensitive noses, have been trained in recent years to pick up the scent of explosives on people moving through crowded concourses,

And so far, they have proved a better early warning system than anything engineers have come up with.

Just this week, the New York City Police Department graduated of squad of these dogs.

But experts say there are not enough of these animals to go around. Only about 130 dogs have gotten the patented training nationwide since its development about a decade ago. And only one dog is in Europe, according to the chief trainer.