They can--if they are allowed to.  

E.J. Dionne, Jr, WaPo last night:

In a process already saturated with cynicism and soured by bad faith, it would be shameful but unsurprising if an FBI investigation aimed at shedding light on Christine Blasey Ford’s accusations against Supreme Court nominee Brett M. Kavanaugh became the object of political manipulation.

The probe is happening because Republican Sens. Jeff Flake of Arizona and Lisa Murkowski of Alaska grasped how hypocritical it would be for their party to claim to take Ford’s heartfelt and credible testimony seriously and then barrel to a confirmation anyway.

But the FBI may be blocked from doing its job properly. Will it be able to provide a basis for judging the relative trustworthiness of the only two witnesses who appeared before the Senate Judiciary Committee last week?

An investigation sharply constrained by White House Counsel Donald McGahn — who is overseeing the FBI’s work and just happened to be Kavanaugh’s leading promoter — would turn a brief moment of limited bipartisanship into a new occasion for rage and recrimination.

The good news is that the investigation offers time for one important reality to sink in: It is simply not true, as was so often claimed, that both witnesses were equally “believable.”

Ford answered every question she was asked and without pretense. Her candor about what she could not recall added immensely to her credibility.

Judge Brett M. Kavanaugh takes a seat before the Senate Judiciary Committee on Thursday. (Matt McClain/Pool/The Washington Post)

By contrast, as Sen. Cory Booker (D-N.J.) noted on Friday, Kavanaugh’s “testimony and claims regarding his drinking habits and behavior in high school and college [and] beyond have been contradicted by so many people. His statements have just proven to be not true.”

Every undecided senator should read Philip Bump’s extensive fact checkin The Post flagging answers Kavanaugh gave that “stretched or misrepresented the truth.” Then, the senators should turn to the New York Times’ equally comprehensive analysis describing responses Kavanaugh gave that were “misleading, disputed or off point.”

.....

https://www.washingtonpost.com/opinions/can-the-fbi-do-its-job-with-kava...

This White House obstruct justice?  Prevent truth from coming out?  Seek to rig investigations they cannot prevent? 

Nah.  Not possible.

I suspect he published this to buck up the FBI troops to stay independent of political pressure and for them tro risk even pissing off the commander in chief if necessary, to stay loyal to their oath to the Constitution. It was at the risk of fueling "deep state" suspicions so I don't think he did it without considering that danger, judged it worth it. I notice he is very careful with what he communicates, very selective statements, tweets infrequently. For myself, makes me feel better about the investigation not being affected by pressure from outside.

(The problem might be more like: they'll just do their same old lousy incompetent job unaffected by politics. I'm actually not a big G-man fan, you see.)