At Dulles International Airport just outside Washington, D.C., the Transportation Security Administration puts Woody, a black Labrador retriever, to the test as a bomb-sniffing dog. Woody was born in early 2010, and weighs 64 pounds. He was trained at the Auburn University Canine Detection Training Center in Oxford, Alabama.

On a recent Friday, Woody patrolled as hundreds of people come through a security checkpoint.

One of these people had explosive material hidden in his luggage as part of the test. Woody sniffed, and locked in, catching the bait. Good dog.

But it doesn't always work this well.

ABC News

A Government Accountability Office report released last week suggests that the Transportation Security Administration is not collecting enough detailed information to know if its bomb dogs are well-trained and capable of finding bombs at the nation's airports. The report includes secret video that shows the dogs failing tests to detect explosives. Sometimes trained dogs miss the target, and instead track an innocent passenger.

"You're using dogs in ways they have not been previously used," said Jeff Price, an associate professor at the Metropolitan State University of Denver who is the author of "Practical Aviation Security."

Dogs have long proven to be very successful at sniffing out explosives in luggage. But now they are increasingly being used to patrol busy airport terminals, sniffing passengers. The GAO report criticizes the government for spending $100 million on a new program, growing to 700 canine teams in roughly a year, before knowing how well the dogs could do the job.

"A lot of money gets spent before they know something works," said Price.

A dog's nose is thousands of times more sensitive than a human's. Dogs are able to sniff items from miles away. But in an airport terminal there may be hundreds of smells the dog must sift through. It can be overwhelming.

Sources tell ABC News in a majority of tests dogs do catch the explosives, but they also routinely fail - though the actual number is classified.

"There's no perfect layer of security," said the TSA's Chris McLaughlin. "T hese dogs are very effective."

The report calls for more testing to ensure that this critical tool in our airport defense is being used in the best way possible.