USGS FAQ:

http://www.usgs.gov/faqs/how-did-snakehead-fish-get-united-states

On a side note, some religions practice a form of animal "mercy" release and there have been a few incidents where people unknowingly released non-native species into US waterways. I think this could be one of the ways the snakehead got here.

https://www.nationalgeographic.com/animals/article/wildlife-watch-mercy-release-buddhist-china-illegal-trade

I know of one that occurred in Berks county that was (luckily) contained to a small pond.