Whatever anyone tells you, Presidential pardons are criminal and invalid if granted for corrupt purposes. In 2001 Bill Clinton pardoned the fugitive Marc Rich. @FBI subsequently investigated the pardon to see if it was a crime. (They closed the investigation in 2005, no charges.)

So when various talking heads say ‘too bad, the pardon power is unlimited under the Constitution’ just remember that is factually false; a lawful act done for an unlawful purpose becomes unlawful; and not only is there precedent to investigate that, there is recent precedent.

Do you know what else is unlimited? The President’s power to fire the FBI Director. But Trump’s firing of @Comey was nonetheless investigated as a crime, because a lawful act (the firing) would have become unlawful if done for an unlawful purpose (obstructing justice).

Many of these pardons are being granted for corrupt purposes. If @FBI, who have a pretty solid view of a lot of criminality not yet made public, think any of these pardons were granted to obstruct justice, or for bribes (spoilers: yes and yes), of course they will investigate.