There are statute of limitations involved here, and it is one of the obstacles the Feds would have to overcome to get a conviction. The general statute of limitations for Federal crimes is 5 years. That applies to mail and wire fraud. Major fraud against the US government is 7 years, and fraud against a financial institution is 10 years.

There are exceptions however to these limitations. Most notably, the fact that a fraudulent scheme extends before the limitations period does not preclude prosecution for acts committed in furtherance of the scheme within the limiations period.

Long winded answer to say there's a statue of limitations problem, but it may not be insurmountable.