Click image for more photos of bomb dogs. Above, TSA explosives detection handler Diego Villanueva and his dog Joe train at the Atlanta airport in December. (AP/John Bazemore)

The now ubiquitous surveillance image of the Brussels suicide bombers strolling through the airport corridor is enough to instill fear in anyone with plans to fly.

In the U.S., security officials are partly depending on bomb-sniffing dogs to thwart similar terror plots.

Since the 9/11 attacks, the number of canines deployed to protect the nation’s busiest airports, train stations and other transit centers has surged 400 percent.

A similar strategy is employed across the world. But the global war on terror’s ever-increasing reliance on man’s best friend is presenting a new problem — a deficit of high-quality bomb dogs.

“More developing countries are incorporating detection dog teams into their national security plan,” Cynthia Otto, executive director of the Penn Vet Working Dog Center at the University of Pennsylvania, told Congress earlier this month at a hearing on canines used for homeland security. “The demand for detection dogs has increased to the point that the quality of dogs has suffered and the price has increased dramatically.”

SLIDESHOW – Bomb-sniffing dogs at work>>>

No agency outside of the U.S. military employs more bomb-sniffing canines than the Transportation Security Administration.

“It’s our best asset out there to find explosives,” Henry Sergent, chief of the TSA’s National Explosives Detection Canine Team Program, told Yahoo News.

This year, more than $120 million is budgeted for the TSA to position nearly a thousand bomb-sniffing dogs and their handlers at transit hubs.

But according to a recent federal audit, the effort is currently short 210 detection teams — in part because they don’t have enough dogs.

Source: Transportation Security Administration

A TSA spokesman said the exact deficiency wasn’t known, but for the first time since 9/11 the agency is seeking to purchase privately trained dogs for possible use.

“We’re just thinking ahead and making sure that we cover our bases so that we have dogs available for any sort of purpose or need that comes up,” Sergent said. “We don’t want to be caught in any sort of position of where … we don’t have an adequate supply.”

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But that problem already exists, according to some veteran canine trainers, academics and law enforcement officials. The market for labradors, Belgian malinois, vizslas and other breeds that make good explosives detectors is so competitive that some trainers wouldn’t reveal their suppliers to Yahoo News.

“There is such a demand for high-quality labradors in the United States that we have to look outside of the United States for labs as well,” said Sue Kjellsen with K2 Solutions, a North Carolina private contractor that has made tens of millions in recent years supplying and training dogs to spot improvised explosive devices (IEDs) for the U.S. military.

Three weeks ago, the Senate Homeland Security and Governmental Affairs Committee summoned Otto, the Penn veterinarian and canine researcher, along with representatives from the TSA, Border Patrol and the Government Accountability Office to discuss how federal dog programs contribute to keeping Americans safe.

“At the same time, these valuable tools are not free,” Sen. Tom Carper, D-Del., said in his opening statement. “Dogs with the proper abilities and temperament to conduct searches are expensive to buy and even more expensive to train and deploy effectively. As with all of our security investments, we must make sure we are deploying these canine teams in a cost-effective way.”

Otto warned about the U.S.’s dependence on importing detection dogs from Europe and, more recently, South America.

“The risks of relying on foreign sources of dogs to support our national security are high,” she testified. “If there was a problem, an interruption in our supply, we would be in big trouble.”

Eastern Europe in particular has been a popular pipeline for decades because dogs there are historically bred for police and other detection work.

“In [the U.S.], we tend to breed dogs for pets and for show, and those are not the same kinds of dogs that we need for this kind of work,” Otto told the committee.

Dog vs. machine

All canines have superpowered snouts — their sense of smell is 40 times greater than a human’s — but the best bomb dogs must also be sociable, disciplined and physically resilient.

“Rriverso,” a labrador named after a 9/11 victim, demonstrated his skills at the homeland security hearing by screening four congressional staffers walking through the room. In no time, Rriverso alerted on a man carrying a briefcase with explosives and was rewarded with his favorite tennis ball.

“There’s no machine that can detect the presence of explosive materials the way that a canine can,” Doug Timberlake, a TSA explosives detection handler, told the committee. “Machines can confirm the presence of explosive substances but they cannot reason and problem solve to find the source of the substance.”

Nor are they as mobile.

Advances in detection training are also propelling the demand for more dogs, whose olfactory senses can detect parts per trillion of an explosive. Until a few years ago, canines were only taught to search for inert objects like cars and cargo while leashed.

But a growing number of dogs are working off-leash in hopes of identifying a suicide bomber afoot, like the terror suspects seen pushing carts through the airport in Brussels. The dogs stay near their handlers, yet can canvass larger venues like football stadiums faster.

“Our dogs can search about 200 people a minute,” said Kjellsen of K2 Solutions, which recently contracted with the Army for further off-leash research and development.

Best breed for the job?

Attrition due to a canine’s age (their careers average eight to 10 years), health or poor performance means the TSA must replace 100 or more dogs a year.

The TSA refers to its dogs that search independently as passenger screening canines (PSCs). Until recently, federal employees trained all TSA canines at the agency’s dedicated facility at Lackland Air Force base in San Antonio.

TSA Terms and Conditions for Trained Canines

But last fall, the TSA began seeking U.S. suppliers for trained and untrained sporting breed labrador retrievers and vizslas capable of passenger screening.

“We’re going to try that,” Sergent told Yahoo News. “Whether it works out or not, we don’t know. We train our dogs to a specific method and quality and standard.”

A purchase agreement, which doesn’t obligate the government to buy, was awarded to three suppliers seeking $8,000 to $21,000 for untrained dogs over a five-year period, “with pricing dependent on when the order is placed and how many canines are ordered,” explained Kurt Allen, a TSA contracting officer.

“Specialized (trained) canine pricing will likely be higher … but by how much is not yet known,” he said.

For dogs abroad, Sergent said that the TSA partners with the Department of Defense and spends about $5,000 per dog. But others told Yahoo News that prices sometimes climb to more than $10,000 per dog if the animal has little or no training.

The TSA’s solicitation to private U.S. dog trainers came with 30 pages of terms and conditions, including a provision that stated “springer spaniels, brittany spaniels, and border collies are not acceptable.”

“Quite frankly, it’s some of the most ridiculous stuff I’ve seen them put out yet,” L.E. Papet, a longtime Ohio dog trainer, said of TSA’s requirements. “There’s kind of a running joke with the feds, for every pound of dog you supply, you’ve got to supply an equal pound of paper.”

If supply and demand is at risk of jeopardizing national security, Papet suggests the TSA abandon what he calls their “cookie-cutter” approach to training and be more open to other breeds that are used for bomb detection in England and elsewhere.

“One of the best sporting breeds out there is the springer spaniel that’s a kickbutt little detection dog,” said Papet, owner of K9 Resources and co-editor of the forthcoming book Canine Olfaction Science and Law. “You need a trainer that will adapt to the dog, not a dog to adapt to the trainer.”

English Springer Spaniel Connie, an explosives detection canine, listens to a command by her handler British Transport Police constable Taff Evans during 2012 Summer Olympics in London. (AP/Charles Krupa)

Sergent said the TSA considers Spaniels to be too short.

“I can train a dog for a lot of things, but just like humans, some things you’re not going to overcome and height is one of them,” he said. “If they don’t get to the odor, they’re not going to detect it.”

As the need for bomb dogs escalates, the University of Pennsylvania’s Otto is pushing for a coordinated national breeding program and more collaboration between U.S. government agencies, academics and the private sector.

“This is the problem with trainers, the only thing they agree on is that the other guy is doing it worse,” she told Yahoo News.

The demand includes more than just airport terminals. A year ago, bomb dogs were hired to patrol the Mall of America after an al-Qaida affiliate called for attacks on the mammoth Minneapolis shopping center. And terror plots in Boston and San Bernardino have police departments across the country considering employing more than just drug dogs.

“Ten years ago having an explosive-detection canine was maybe a nice thing to have for a security program, but it should be a standard … if you are trying to address today’s threat,” said Michael O’Neil, a retired New York City police commander and a founder of the NYPD Counterterrorism Division.

O’Neil is now CEO of MSA Security, a private firm that provides protection for corporate and government clients. Following the San Bernardino terror attack, MSA was training canines day and night.

“We’re working some overtime so that we can have more dogs out in the field,” he said.

Jason Sickles is a national reporter for Yahoo News. Follow him on Twitter (@jasonsickles).