Why is an FBI investigation of former CIA Director Gen. David Petraeus still going on?

Eli Lake asked the question in a new report for Bloomberg View and sat down with Gretchen Carlson this afternoon to discuss the legal situation of one of the nation's military heroes. 

Lake noted that Petraeus was hired by private-equity firm KKR, has academic positions at Harvard and USC, and was reportedly consulted by President Obama on how to fight ISIS. 

Yet the 2012 probe into whether he improperly gave classified information to his mistress still hangs over his head.

Gretchen Carlson asked about the possibility that the administration is using the investigation as a way to keep Petraeus from speaking publicly.

In a letter, Sen. John McCain (R-Ariz.) called on Attorney General Eric Holder to end the investigation one way or the other. 

"Without commenting on the underlying merits or anticipating the outcome of the investigation, I can conclude this is a circumstance in which the principle of "justice delayed is justice denied" is certainly at play, and that this matter needs to be brought to resolution. Furthermore, the fact that you and others within your Department have weighed-in publicly on the case raises questions about whether this investigation is being handled in a fundamentally fair and appropriate manner," McCain wrote.

Lake noted that even Obama said at the time that the information given to Paula Broadwell was not harmful to national security. 

"So what's the motive?" Gretchen asked. "Do they want to keep him quiet on something?"

Lake said he's still reporting on the situation, but added that Petraeus' allies "have suggested very strongly that because this is hanging over him, Petraeus has been very circumspect in his public comments." 

Gretchen pointed out that since resigning, Petraeus' comments about the Obama foreign policy have been mostly positive. Meantime, former Defense Secretaries Robert Gates and Leon Panetta have been highly critical. 

Watch the full discussion above.