my only question is "why at a later date?"

Biden suggests bringing criminal charges against Bush admin.

In a recent interview with Newsweek, Sen. Joe Biden (D-DE) said he is opposed to impeachment proceedings because they would disrupt Congressional business. “But he has a pretty stunning Plan B“:

The alternative, and it’s taken me time to think through, I think we should be acquiring and accumulating all the data that is appropriate for possibly bringing criminal charges against members of this administration at a later date.

(Think Progress)