They're America's cutest assassins with a federally-protected status. At least that’s how some scientists refer to the black-footed ferret, or the BFFs, for their ability to kill prairie dogs.

According to the U.S. Fish and Wildlife Service, BFFs are on a federally endangered list. Human development and diseases have nearly killed all the animals at one point.

The U.S. Fish and Wildlife says BFFs were believed to be extinct twice in the 20th century. In 1981, a small population was found in Wyoming. But in 1986, only 18 were “known to exist in the wild.”

Scientists captured those animals and they became the foundation for a breeding program.

The U.S. Fish and Wildlife’s National Black-Footed Ferret Conservation Center in Northern Colorado works with many partners in multiple states to help rebuild the BFF’s population.

“Ecosystems are delicate systems and you need predators to maintain a balance,” said Ryan Moehring, public affairs specialist of the U.S. Fish and Wildlife Service Mountain-Prairie Region. “So when you have a keystone predator like a black-footed ferret, it keeps other species populations in check, like prairie dogs and they also provide prey for other species, raptors, prairie rattle snakes, that sort of thing.”

There are currently about 300 BFFs living in captivity and about 400 to 500 in the wild nationwide.

“Ferrets are a really special species. They’re not only extremely cute, they’re ferocious,” Moehring said. “Fun fact – they have the largest K-9 teeth in proportion to their body of any species on the planet. These use those teeth to capture and kill their prey. They’re this really interesting balance between cuddly and deadly.”

In 2015, the U.S. Fish and Wildlife Service released 25 ferrets at the Rocky Mountain Arsenal National Wildlife Refuge. Once a year for two weeks, the scientists survey how many animals survived at the Arsenal and if any were born in the wild.

A 9NEWS team is the first TV crew tag along overnight.

We'll have the story on the networks of 9NEWS Wednesday.

Oh - there's also a black-footed ferret cam, and a Facebook page!