A State Department official offered a "quid pro quo" deal if the FBI would change the classification of a Hillary Clinton email, FBI documents indicate.
Patrick Kennedy, an undersecretary of state, had asked the email be downgraded to a lower category.
In exchange, an FBI agent said, Mr Kennedy offered to accept an FBI request for extra agents at foreign diplomatic posts.
Both the FBI and the State Department deny any deal was offered or struck.
State Department spokesman Mark Toner said there was no "quid pro quo" and it was the FBI official who raised the issue of putting more agents in Iraq, when they discussed the email.
Other interesting revelations from the newly released FBI documents include:
- A former diplomatic security agent said Mrs Clinton would often "blatantly" disregard security and diplomatic protocols, including her frequent refusal to attend overseas diplomatic functions with local ambassadors, which left envoys "insulted and embarrassed"
- While secretary of state she ignored security advice by going to a dangerous part of Jakarta, Indonesia, in a move that "placed Clinton, her staff, the media and her security detail in unnecessary danger in order to conduct a photo opportunity for 'her election campaign'"
- A group of top State Department officials that some called "The Shadow Government" met weekly to discuss Freedom of Information requests related to Mrs Clinton. They wanted her emails to be released all at once, instead of on a rolling basis, as would normally be the case, according to the FBI summary. But the group did not get its way.
- Explaining that security and protocol procedures were regularly breached, the report states that early in Mrs Clinton's tenure as secretary of state, "she and her staff were observed removing lamps and furniture from the State Department", which were brought to her residence in Washington DC. The unnamed source does not know if the government property was ever returned.
The 100-page FBI document published on Monday says an unnamed FBI official was "pressured" in summer last year to change the classified Clinton email to unclassified.
The official said he had received a call from a colleague - whose name is also redacted - at the FBI's International Operations Division.
The colleague said he had been contacted by Mr Kennedy asking him to change the email's classification level in "exchange for a 'quid pro quo'".
The State Department, the document says, offered to "reciprocate by allowing the FBI to place more agents in countries where they are presently forbidden".
Mr Kennedy, according to the document, followed up on this matter at a subsequent meeting, spending 15 minutes attempting to influence the FBI to unclassify the Clinton email.
He also pressed FBI assistant director of counterterrorism Michael Steinbach on the issue.
Republican House of Representatives Speaker Paul Ryan said the revelation "bears all the signs of a cover-up".
Republican presidential candidate Donald Trump tweeted: "Unbelievable."
But Mr Toner denied there had been any wrongdoing, and said Mr Kennedy was merely questioning why the email was categorised as such.
"Under Secretary Kennedy sought to understand the FBI's process for withholding certain information from public release," Mr Toner said.
"As has been reported, there have been discussions within the interagency on issues of classification."
The email in question was among some 100 messages recovered from Mrs Clinton's private email server that the FBI would ultimately designate as classified.
The FBI and State Department engaged in a review of the former secretary of state's emails as they were being made ready for public release.
FBI Director James Comey concluded in his investigation Mrs Clinton had been "extremely careless" in handling the classified material.
The emails scandal relating to her private server is different from the messages that Wikileaks have been releasing recently.