Section 9 - Limits on Congress

No Bill of Attainder or ex post facto Law shall be passed.

Ex post facto

ex post facto adj. Formulated, enacted, or operating retroactively. [Med Lat., from what is done afterwards] Source: AHD

In U.S. Constitutional Law, the definition of what is ex post facto is more limited. The first definition of what exactly constitutes an ex post facto law is found in Calder v Bull (3 US 386 [1798]), in the opinion of Justice Chase:

"1st. Every law that makes an action done before the passing of the law, and which was innocent when done, criminal; and punishes such action. 2d. Every law that aggravates a crime, or makes it greater than it was, when committed. 3d. Every law that changes the punishment, and inflicts a greater punishment, than the law annexed to the crime, when committed. 4th. Every law that alters the legal rules of evidence, and receives less, or different, testimony, than the law required at the time of the commission of the offense, in order to convict the offender."

So according to that those people who were involved in an action before the law passes aren't to be punished.