We all love to talk about an "October surprise" in an American election.
Well, today we got it. More an October shock.
Will it change the outcome of the race? Way, way too early to say that. But it just could.
The FBI announced in a letter to Congress that it's reopening the investigation into Hillary Clinton's emails. The old question that has haunted her for months - was she criminally negligent in her handling of classified information? - has come back with a vengeance.
But let's be clear. There is much we don't know:
Has the FBI stumbled on some incriminating email chain that shows she knowingly misled investigators or deliberately deleted damaging information?
Is it a game changer? Did she exchange - for example - the names of covert CIA officers over her unsecured private email system?
Will the FBI tell the American voters - with just 10 days to go - what's behind the decision to reopen the investigation? If not, the next week will be a maelstrom of speculation and feverish journalistic investigation.
How will Hillary Clinton respond to this late-breaking crisis? Does she even know what is the subject of the new FBI probe?
How did the FBI get this new info? If it came via Wikileaks - and ultimately from the Russian hack of the Clinton emails - that means the Kremlin can be accused of manipulating the US election campaign.
So, let's say this: It's potentially very damaging. Probably, it won't swing the race (remember 20 million Americans have already early-voted), but it's wonderful news for the struggling Trump campaign. And - as I hope I've made clear above - we have tonight many more questions than answers.
ITV News Washington correspondent Robert Moore reports: