Nor was there one for the movie. The Coens have admitted that the "true story" statement at the beginning of the movie was itself a dramatic device, intended to help people see everything in a more realist way. At the end of the movie there was the usual "this is a work of fiction, any similarity to real persons living or dead is purely coincidental" disclaimer.

That said, there were apparently some real-world events that inspired elements of the story in the movie, they just weren't connected with each other, or in the same geographic area, and many events in the move were completely fabricated.

For the show, I'm pretty sure none of it has any basis in reality. I think it was actually a mistake to put the "true story" cards at the front of each episode of the TV series. For one thing, this is really implausible as a true story, whereas the movie wasn't.