I don't know that he's actually "trustworthy" and certainly not "because some Senators say so". I'm of the belief that, because of his predicament, that he may be, ahem..."very cooperative".
He may be holding a whole lot of evidence, both damning and exculpatory, and either unload it, or lose it...depending on what happens to him.
And as I've said repeatedly, you don't allow criminals to run criminal investigations, especially not investigations of things they were personally involved in, or in investigations they are believed to have falsified before. It would be much too easy for them to cover up their own criminality, if not the criminality of others, by being in charge of it again.
Instead, you charge them, then get them to spill their guts on what they know, if they want any leniency. Then you give whatever they said to a new and trustworthy investigator, to follow up on, and question them again if necessary in the future.
Which of those two options should be preferred, if you wanted to start a new investigation, do you think?