Democratic presidential candidate Hillary Clinton met with federal investigators probing her use of a private email account while serving as secretary of state, according to her campaign. (Reuters)

The FBI interviewed Hillary Clinton for three hours Saturday, the presumed final step of its investigation into whether the former secretary of state or any of her top aides exposed government secrets because of her decision to exclusively use a private email server while serving as the nation's top diplomat.

Assuming that the long-awaited Clinton interview does indeed signal the imminent end of the investigation, the timing could get very interesting, very quickly.

The Democratic Party's national convention opens in Philadelphia on July 25 — 23 days from today.  Until Clinton is formally nominated by the convention delegates, she is only the presumptive nominee. And depending on when the FBI decides to announce its finding, it could badly complicate Clinton's attempts to build momentum going into and coming out of the convention.

There has long been an assumption in political circles that the FBI would need to interview Clinton and make public the findings of their investigation before the convention. The reason? If Clinton was indicted for her role in creating and maintaining her private email server, she would almost certainly be forced to leave the race. Nominating someone still under an FBI investigation seemed like a massive risk.

Assuming that logic is right, then the next two weeks will be critical for the presidential race. The FBI won't announce anything Sunday or July 4. Which means the agency will have between July 5 and July 25 to make public its decision on the case. That's not a long time. (Side note: I think it is very unlikely the FBI would choose the Republican convention, which opens July 18, to close the investigation. If that's right, then the Justice Department has even less time.)

All of this is moot if the FBI finds that Clinton did nothing criminal in the email controversy. Republicans will still push it as an issue, but for most of the country it will be considered a settled matter.

However, if there is an indictment or even a harsh scolding in which the Justice Department implies Clinton knowingly and purposely skirted the law, the timing of all of this starts to matter. A lot.

The closer the announcement comes to the start of the Democratic convention, the harder it is for Clinton to control. Clinton's goal throughout this investigation has been to insist that she is totally innocent in this, that the entire email "controversy" is a Republican witch hunt enabled by the media.

If, suddenly, the Justice Department of a Democratic administration shattered that story with just days left before the convention, the negative momentum it would cause might make it tough for Clinton to recover. There would be doubts bordering on panic about What It All Means for Clinton going into the fall campaign, and she would have very little time to turn the story back in her favor.

Why does all of that matter? Because — as any Bernie Sanders supporter will tell you — Clinton doesn't have 2,383 pledged delegates: She has 2,220. This means she needs unpledged superdelegates to put her over the top. If there are major doubts about Clinton's ability to win in November, there could well be a major move of superdelegates away from her. But to whom?

To be clear, Clinton remains, by far, the most likely nominee for Democrats. But, the uncertainty of both when the FBI will make its findings public and what they will say makes the next two weeks the most unpredictable and pivotal of the 2016 election.