TSA officers at BWI Marshall Airport were surprised when they saw a dog's moving skeleton appear on their X-ray review screen Friday morning.

WASHINGTON — A man traveling through BWI Marshall Airport sent a dog and handgun through the X-ray machine at the security checkpoint.

The man, whose name was not released, was arrested Friday morning on Maryland gun charges, the Transportation Security Administration said.

The small dog was inside a pet carrier that looked like a duffle bag as it moved along the conveyor belt and through the X-ray machine. Security agents didn’t know it held an animal until they saw the dog’s moving skeleton show up on their screen, the TSA said.

Screeners found the 9 mm handgun in a gun case plus two empty magazines in a carry-on roller bag. The traveler was also carrying a small knife.

Passengers traveling with small pets should remove the animal from its carrier and hold their pet as they pass through the metal detector, the TSA said. Pets should not go through the X-ray machines.

Guns must be unloaded and packed in checked bags and declared to the airline.

Learn more about flying with guns here. And the TSA has also posted tips for pet owners here.