A senior State Department official tried to strike a cover-up deal to bury one of Hillary Clinton's emails, newly-released FBI documents suggest.
Controversy over her use of a private email server has dogged Clinton throughout her campaign and this latest revelation could prove damaging, with just two days until the final presidential debate and 21 days until the election.
Here's everything you need to know.
- So what's this all about?
The FBI released a batch of new files from its investigation into Clinton's private email system on Monday.
The most damaging aspect for Clinton is an allegation from an unnamed FBI employee who claims that Patrick Kennedy, the State Department's under secretary of state for management, reached out to the bureau in the summer of last year to ask for an email to be changed from classified to unclassified as part of a "quid pro quo" (an exchange of goods or services).
The email was in relation to the 2012 attack on a US compound in Benghazi, Libya which left Ambassador Christopher Stephens and three other Americans dead.
The alleged deal would have allowed FBI agents to operate in more countries.
The conversation is said to have occurred during the investigation into Clinton's use of emails during her time as secretary of state, and as intelligence officials were discussing which of her emails could be made public.
- How have the FBI and State Department reacted to the claim?
State Department Deputy Spokesperson Mark Toner strongly denied any deal had been set up, saying the allegation is "inaccurate and does not align with the facts".
The FBI said the allegations were investigated but insisted there "was never a quid pro quo" or improper activity.
- And Mr Trump?
Donald Trump reacted instantly, tweeting: "Unbelievable".
He later tweeted this video statement:
Trump's running mate, Mike Pence, told voters at a rally in Mason, Ohio that the claims showed why Clinton "must never be allowed to serve as president of the United States of America".
House Speaker Paul Ryan, the most senior elected Republican, said it bore "all the signs of a cover-up".
- Remind me, what's the deal with Clinton's emails?
Clinton set up a private email server from her home in New York when she became secretary of state in 2009.
She used this server and the email address email@example.com, rather than a government email, for all her electronic communications, both work and personal.
Clinton insists the server was purely for "convenience" as she did not want to carry two phones, one with a personal email and one with a work email, but her critics claim the system was designed to give her complete control of her records and put classified material at risk.
The FBI launched a year-long investigation into her email practices but did not recommend criminal charges.
- How damaging is this for Clinton?
It's not ideal.
The email scandal - and winning back the trust of Americans - has been one of Clinton's biggest challenges throughout her campaign and this latest allegation may deepen suspicion around her at a time when she needs to be making a final push for votes.
It also gives Donald Trump - who has vowed, if elected, to appoint a special prosecutor to investigate Clinton and send her to jail - fresh ammunition for Wednesday's final debate in Las Vegas.
The seemingly never-ending saga and WikiLeaks' continued push to release hacked emails from her campaign chairman is a distraction Clinton did not need as she seeks to take advantage of a tumultuous few weeks for her rival.