Q What has FBI director James Comey revealed about the e-mails?
A He sent a letter to Congress last Friday that said agents had uncovered new e-mails that might be connected to the Clinton investigation. That investigation had examined whether Mrs Hillary Clinton and her aides had mishandled classified information by sending it through Mrs Clinton's private e-mail server. The inquiry was completed in July with no charges filed.
Mr Comey said agents would review the new e-mails - which The Wall Street Journal said numbered about 650,000 - to see whether they contained classified information.
The letter was sent 11 days before the presidential election and it set off fierce criticism of Mr Comey for appearing to meddle in politics.
Q Where did the new e-mails come from?
A Mr Comey did not say in his letter. But law enforcement officials briefed on the investigation said agents had discovered the e-mails on a laptop owned by Mr Anthony Weiner, a disgraced former congressman and estranged husband of Mrs Clinton's top aide, Ms Huma Abedin.
In September, the FBI began investigating allegations that Mr Weiner had exchanged sexually explicit messages with a teenager. On Oct 3, agents in New York executed a search warrant to obtain Mr Weiner's iPhone, an iPad and the laptop. Searching the laptop, they found evidence of a trove of e-mails similar to ones that had been examined in the Clinton investigation.
On Sunday, the FBI obtained a new warrant to search the e-mails to determine whether they contained national security information.
Under federal law, mishandling national security information is a crime, one that the FBI is responsible for investigating.
Q What does all of this mean for Mrs Clinton and her campaign?
A It is not yet clear. Polling on weekends can be unreliable, so it may be a few days before the impact of the development can be fully assessed.
What is evident is that a campaign that has largely been a referendum on Mr Donald Trump - particularly since the first presidential debate - is now not so clear-cut. The e-mail development will certainly matter but the question is just how much.
Early voting is already under way and 21 million people have voted.
Q What happens now?
A In the coming days, the FBI will begin conducting a smaller version of the investigation it completed in July. Many of the newly discovered e-mails are likely to be duplicates of others the FBI has already examined, investigators say.
The bureau will again look at the question of whether anyone intentionally committed a crime.
FBI agents are all but certain that the review will not be completed by Election Day on Nov 8, and believe it will take at least several weeks.