This is one of those generational stories...  You have to be at least my age to really remember Jimmy Hoffa (I was a month shy of turning 14 when he disappeared). In the same way my parents understood a reference to Judge Crater and I didn't,  the under 50 crowd today knows little about Hoffa's life or death. My favorite rumor was that he was underneath one of the Giant's Stadium end zones. 

If the FBI believes this guy credible, you would expect them to react to this, and soon.