Levi seemed like the perfect gentleman. On a recent humid morning in Saco, the 3-year-old German shepherd posed for pictures with his owner, happily wagging his tail in the early-morning sun. Afterward, he cozied up to several bystanders, letting them scratch his shiny, black-and-tan coat.

But when Levi’s trainer yelled “revier!” a German dog-training command meaning “search,” the lean, sinewy dog exploded through the training course.

He galloped around every impediment, searching for intruders. When he found one, he cornered him, bared his razor-sharp teeth and barked incessantly.

For a few seconds, the intruder froze. And when the intruder tried to run, Levi sunk his teeth into the intruder’s heavily-padded arm. The dog didn’t let go — despite whacks from the intruder’s stick — until Levi’s trainer yelled “platz!”

Within seconds, Levi reverted to the family-friendly, picture-posing pup from just a few minutes earlier.

“That’s the beauty of them,” said Doreen Metcalf, Levi’s trainer and head of the Maine Schutzhund Club. “They’re great with people, and they’ll keep you safe.”

Schutzhund — which means “protection dog” in German — is a sport that measures the physical and mental capabilities of police-type dogs. It consists of three phases: tracking, obedience and protection.

Levi and others like him have many of the same skills as the dogs used in elite military units like Seal Team 6, which took a dog on the May raid of Osama bin Laden’s compound. One of Metcalf’s dogs now works for the Bangor Police Department.

Schutzhund emphasizes controlled aggression and top-notch people skills over the wild ferocity sometimes exhibited by other animals.

Schutzhunds, however, don’t come cheap. While a German shepherd from a pet shop or breeder will often go for a few hundreds dollars, Metcalf sells well-trained dogs to breeders, police departments and individuals for as much as $7,000.

She also trains others’ dogs for $1,000 per month. A woman from New York just sent her two German shepherds — Max and Ruby — for two months of training.

The dogs can go for much higher prices, too. If the dogs have good bloodlines and excellent hips, and if they score in the 90s on a 100-point scale at national competitions, they can go for as much as $20,000 or $30,000, Metcalf said.

Wayne Curry, who trains high-end dogs near Olympia, Wash., concedes the price tag is steep. But for wealthy Americans like top executives or entertainers, the price is justifiable, even for the most expensive schutzhunds.

“When you compare the costs of a full-time bodyguard versus a dog, the dog makes a lot of sense,” Curry told The New York Times. “And the dog, unlike the bodyguard, can’t be bought off.”

In Saco, at the only schutzhund club in the state, the owners and Metcalf don’t deal with the highest-priced dogs. Metcalf is more interested in training happier dogs, she said, than the ones who score in the high 90s at national and international competitions.

Michael and Carol Crosby, whose two Doberman pinschers — Paige and Argos — also train at the club, aren’t in it for the money, either. Their dogs love schutzhund. It keeps them “well-balanced, happy and fit,” Michael said.

For some people, a gun or security system makes more sense, club members said. But for people who don’t like guns, or who want companionship in addition to protection, schutzhunds are an excellent option.

“If you’re a woman jogging in the park at dusk, it’s reassuring to have one of these guys with you,” Metcalf said. “You’re not a target if you’ve got something like this beside you. You’re just not.”