They heard his remarks. They were quite plain. Why do they need a probe?
The Dems want the country to grow completely weary of the long slog toward properly seeking justice. These Dem backbiters want to give the appearance of an endless back-and-forth, to which they hope the populace will soon (upon hearing of FBI/DOJ indictments, for example, just as we might see hints of accountability and justice for all) rise up to say, "STOP! Just make it all stop!"
If they can steer the citizenry to reject PDJT's commitments to hold government criminals to account, by just such ludicrous moves as this attack on AG Barr, then they will have scrabbled ever closer to their totalitarian-socialist goal of enslaving us, we who used to boldly speak of freedom.