alt Hacker NewsEx Parte Garland was decided in 1866
“The power of pardon conferred by the Constitution upon the President is unlimited except in cases of impeachment. It extends to every offence known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken or during their pendency, or after conviction and judgment.”
Nobody is going to waste their time investigating crimes that can't be prosecuted.
You can't pardon violations of State law though. There may be plenty of offenses to go after. Vindictiveness is required at this point.