From Wiki:
Although the film plot is completely fictional, the Coen brothers claim that many of the events that take place in the movie were actually based on true events from other cases that they threw together to make one story.
“although completely fictional” then, “they claim”
Yep, the perfect perpetuation of the hoax....
;-)
I still enjoy it tho and was a bit dissappointed when I learned of their trickery!