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.

But the only ferret species native to the Americas was nearly wiped out by human development, disease such as the plague, and the eradication in some places of its food - the prairie dog.

“The story behind the BFFs is fascinating and it’s storybook,” said David Lucas, Refuge Manager at the Rocky Mountain Arsenal National Wildlife Refuge. “Twice thought to be extinct. Twice thought not to exist on this planet anymore and yet a sheep dog in Wyoming brought one home from a ranch and it started this entire program. A great story.”

According to the U.S. Fish and Wildlife Service, BFFs are on a federally endangered list.

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 canine 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.

As of last year’s count, 50 black-footed ferrets lived at the Arsenal.

“Our long-term goal is for us to be able to breed animals that were born in the wild that can be reintroduced to other sites across the state and across the country,” Lucas told 9NEWS.

Part of the yearly survey is capturing the ferrets at night, checking if they have microchips similar to dogs and cats, updating their vaccines, and releasing them back.

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On Tuesday night, 9NEWS went along on the black-footed ferret survey with the U.S. Fish and Wildlife folks. Nick Kaczor, the assistant manager at the Rocky Mountain Arsenal Wildlife Refuge, was administering vaccines to ferrets that night.

“I love doing it,” he said. “Even though it’s 4 o’clock in the morning, been up for probably 24 or 25 hours now, it’s a great opportunity. It’s very exciting to be this successful with a ferret recovery. We’ve been amazed at how well the ferrets have been doing out here at the Rocky Mountain Arsenal. We’re very excited for the future. We’re very excited to hopefully take the ferrets from this site and help other recovery sites in the future with wild born ferrets.”

Other recovery sites included South Dakota, Wyoming, Montana, Arizona, Utah, Canada and Mexico.

Some zoos across the country also help breed black-footed ferrets.

PREVIOUS: America's BFF: About the federally-protected black-footed ferret

The U.S Fish and Wildlife Service says for black-footed ferrets to be taken off the endangered list, the Service needs 3,000 breeding adults in 30 or more populations in at least 9 of the 12 states across the animal range. The Service estimates it will need 500,000 acres of managed prairie dog colonies to achieve that goal.

The Service says with the existing 28 reintroduction sites and other safe harbor agreements in place, it believes it will eventually remove the black-footed ferret from the endangered list. Oh - there's also a black-footed ferret cam, and a Facebook page!