By Whom? The FBI and DOJ??!!
They are executive branch agencies. The DOJ is run by Jeff Sessions who can order an investigation and prosecution. He can, with a signature, stop or change most of this (not all but most).
I think Trump believes he is best served if this entire issue stays alive and active. Right from The Art of the Deal:
"It doesn't matter what they say about you, as long as they are talking about you"As long as there is the 'witch hunt' it galvanizes his followers behind him. He has positioned his entire marketing gambit on 'he is the people and the people are him' (my phrase)- so as long as there appears to be a witch hunt against him, his supporters will feel as though it is an attack on them and it will strengthen loyalty. Even if he shot James Comey on 5th Avenue, if they tried to prosecute him for that, it would be used the same way.
Like most politicians in the game, it isn't about resolution- that's the last thing they want. It is about keeping the issue alive.