DNA tests have confirmed that three man-eating Nile crocodiles have been found living in Florida's swamps.
Unlike local alligators, the species preys on humans and is thought to be responsible for up to 200 deaths a year at home in sub-Saharan Africa.
It is possible more of the beasts are at large in the state, experts say.
It is not known for certain how they reached the US. "They didn't swim from Africa," said University of Florida herpetologist Kenneth Krysko.
One likely possibility was that they were brought in illegally by unlicensed collectors, who then failed to keep them secured or intentionally released them, Krysko told the Associated Press news agency.
The animals were found in 2009, 2011 and 2014 and were confirmed as Nile crocodiles by a recent DNA test.
The Nile species can grow to up to 6m (20ft), significantly larger than local alligators, which commonly grow up to 4m.
They are known to prey on shrimp, fish, insects, birds and mammals, including humans. They are also known to attack livestock.
Florida wildlife experts are concerned that the African species could pose a threat to the state's ecosystem if they breed in the Everglades wetlands.
The Burmese python was first sighted in the Everglades in the 1980s and there is now an established population of the snake.
"I have two words: Burmese python," wildlife biologist Joe Wasilewski said. "If you would have told me 15 years ago we would have an established population in the Everglades, I wouldn't have believed you."
When alien species attack
Alien wildlife can wreak havoc in an unprepared ecosystem. When the Burmese python turned up far from home in the Florida Everglades in the 1980s it bred fast, sustaining its reproduction by feasting on endangered local wildlife, including alligators. There are now thought to be about 30,000 of the formidable snakes in the area.
But invading species don't always come in large packages. The Indian Silverleaf - or Sweetpotato - whitefly, just a millimetre long, is estimated to have caused more than $100m worth of damage attacking crops across California, Texas and Arizona in the 1980s.
Sometimes the species don't even have to invade, they are invited. Cane toads, native to South and Central America, were introduced to Australia in the 1930s in an attempt to control the grey-backed cane beetle, which was destroying cane crops. But with no natural predator, the poisonous toads spread like wildfire, killing native species as they went.
And invasive species aren't always obviously menacing either. In 1859, Thomas Austin had 24 ordinary rabbits shipped to Australia for hunting purposes. "The introduction of a few rabbits could do little harm," he reportedly said at the time. But Austin underestimated the habit of rabbits to reproduce like, well, rabbits. Soon there were tens of millions and they killed off local plant species, having a devastating effect on Australia's ecosystem.