Tony is right.
The Northern Snakehead is the one causing all the problems and it was never a popular aquarium fish. The Giant Snakehead (the pretty little red one until it turns large and green) was a popular aquarium fish but is tropical and comes from Thailand. Individuals have been found throughout the U.S. (mostly before the federal ban on snakeheads) but the only breeding populations are suspected to be only in South Florida.
Northern Snakeheads come from China and can tolerate cold water. They are the ones that are breeding. They are popular as food fish and were released by fishmongers.