I believe the answer is here: https://en.wikipedia.org/wiki/Ex_post_facto_law#United_States https://en.wikipedia.org/wiki/Ex_post_facto_law#United_State... Co

I believe the answer is here: https://en.wikipedia.org/wiki/Ex_post_facto_law#United_States https://en.wikipedia.org/wiki/Ex_post_facto_law#United_State... Congress is prohibited from passing ex post facto laws by clause 3 of Article I, Section 9 of the United States Constitution.