Major spoiler ahead be advised !

In the movie The Drop, we see how Bob Saginowski (Tom Hardy) who acted as the bartender for Cousin Marv (James Gandolfini) :

in the case of the kid that got the money from the loan shark that is cousin Marv and then paid him back, apparently Cousin Marv didn't appreciate that, and got him killed. However the cops were unable to relate Bob to the murder. And everybody around town were suspecting the bit of lunatic Eric Deeds of the act. However, it is revealed that Eric couldn't possibly be part of the murder since that day he was at a mental institute. Which leads to my question: WHY didn't the cops act on a 10 year old murder with clear evidence that the main suspect is not guilty and the real killer is still on the loose? Am I missing something here? Couldn't they trace back the loan he got from Cousin Marv?