How many banks were involved? There were a lot of people banking money too. So how did the banks know that the money came from a ponzi scheme? Did Madoff tell them it was fraudulent money?
A total outsider, Harry Markopolous was able to spot the fraud just on the basis of what Madoff was reporting as returns. Does it really seem to you impossible that banks who were working with Madoff on a daily business might have suspected fraud?