Hillary Clinton's sole use of a private email account as secretary of state is becoming a persistent, if confusing, issue on the campaign trail, and the fallout could grow still. Josh Robin filed the following report.

Hillary Clinton was asked this week if she wiped her computer server clean of data.

"What, with like a cloth or something?" she said, jokingly.

Her joking belies increasing seriousness over her emails as secretary of state. It's causing not just political trouble, but an FBI inquiry.

It began, she says, as a way to simplify her life with a single email address.

"In retrospect, what was supposed to be convenient has turned out to be anything but convenient," Clinton said.

Not convenient, but, she says, not criminal, either.

Former U.S. Attorney General Michael Mukasey isn't sure.

"If she came up with the idea and gave the order, then it would appear that's responsible under that statute, criminally responsible," Mukasey said.

Now in private practice, Mukasey is listed as an adviser to Republican presidential candidate Jeb Bush. That statute he talks of criminalizes storing classified information at an unauthorized location. A similar situation led to the guilty plea of former CIA Director David Petraeus.

Mukasey says to be charged, documents don't have to be marked classified, which is key because Clinton says hers wasn't.

Mukasey: It's documents containing classified information.
Robin: Even if it's not labeled.
Mukasey: Correct.

However, when it comes to whether something is classified, government agencies sometimes disagree.

Clinton advisers say she's caught amid the varying standards of classification.

But classified or not, what crossed Clinton's desk was sensitive, like the 2012 death of the U.S. ambassador to Libya.

"She doesn't deal with questions of receiving an ambassador's credentials or what somebody should serve at a state department reception," Mukasey said.

That raises questions about why Clinton felt it OK to bank a server based out of a Colorado firm.

There's also the question of whether Clinton only deleted from her server emails relating to non-official business. That's what she insists.

Politically, this investigation is a distraction, to say the least. In all likelihood, it will last months more.

After a testy exchange Tuesday, Clinton didn't appear publicly Wednesday.