Service dogs come in all shapes and sizes, but it's their training that really distinguishes the legitimate ones from fake service dogs.

Nineteen states make it a crime to dress up a pet and pretend it's a working service dog, including the state of Virginia.

Getting caught with a fake service dog may result in a misdemeanor. But the laws don't seem to be dissuading the fraudsters. Currently, there is no national registry to track service animals. Meanwhile, scam artists online are selling fraudulent certification, ID cards and vests. Owners can easily purchase a service dog vest for their pet.

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For example, WUSA9 has found video of an owner pushing her three pet dogs inside a shopping cart. The owner claims them as service dogs. In another instance, an owner wears sunglasses and pretends to be blind, so the pet dog can pretend to be a guide dog inside the grocery store.

When a pet owner poses their dog as a service dog in public, it can harm people who actually rely on legitimate service dogs.

"It's just so selfish. Why would you do that?" questions Sgt. Mike Garvey who has to jump through hoops to prove Liberty is a trained service dog.

Garvey is a Marine Corps veteran who fought in Afghanistan. He was shot twice on tour.

Another veteran became so exasperated by fake service animals, he launched a Facebook page to expose the impersonators. We agreed to disguise his identity because he says he receives frequent threats.

"There is nothing worse than pissing off a combat veteran who fought for his country, and now he has to fight to be able to go inside of a local grocery store," he said.

He has also witnessed the solemn changing of the guard at The Tomb of the Unknown Soldier disrupted by fake service dogs.

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"There were a couple of hound dogs with other people and they began howling at a squirrel," he described. At Dulles International Airport, he and his service dog were stopped by multiple security officers.

"There are so many dogs coming into the airport and relieving themselves, growling, barking and biting they can't tell the difference between a legitimate trained dog and a fake service dog," he said.

Nationwide, the situation has gotten so out of control, even Late Night host Stephen Colbert mocked the absurdity.

"My dog isn't really a service dog. I just bought him a vest online so I could take him to the movies with me," Colbert said referring to his pug.

Meanwhile, Ryan Honick's service dog, Pico, has finished two years of rigorous training. Pico has mastered more than 40 commands.

"He just makes going through my day to day so much simpler," Honick said.

RELATED: What you should know about service dogs

A trained service dog would never:

  • Bark in public, except on command, or be aggressive with other dogs or people
  • Be aggressive toward other dogs or people
  • Eat off a table in a restaurant
  • Ride inside a shopping cart
  • Be dressed in an unprofessional manner.

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