Donald Trump has accused Hillary Clinton of colluding with US authorities to "look less guilty" in relation to her use of a private email system while serving as secretary of state, but the State Department has denied any wrongdoing.
The Republican presidential hopeful seized on newly released FBI documents as proof the State Department pressured the agency to alter its findings in a probe of Mrs Clinton's use of private servers.
The Federal Bureau of Investigation documents released today showed a senior State Department official had asked the FBI to declassify or lower the classification of one Clinton email that had been rated "secret."
Patrick Kennedy, an undersecretary of state, had asked that the email be rated to a less-sensitive category.
In exchange, according to an FBI agent, Mr Kennedy had said the State Department would accept an FBI request for additional agents at foreign diplomatic posts.
In a message posted on social media, Mr Trump described the revelations as "one of the breaking stories of our time".
"This is collusion between the FBI, Department of Justice and the State Department to try make Hillary Clinton look like an innocent person when she's guilty of very high crimes," Mr Trump said.
Two key Republican lawmakers in the House of Representatives called for President Barack Obama to immediately remove Mr Kennedy pending an investigation, saying his actions were "extremely disturbing."
"Those who receive classified intelligence should not barter in it - that is reckless behaviour with our nation's secrets," said Jason Chaffetz, chair of the Oversight and Government Reform Committee, and Devin Nunes, chair of the Intelligence Committee.
In the end, the email was not declassified.
In a statement, the FBI said the request for job posts was not linked to discussions about the email classification.
"The allegation of any kind of quid pro quo is inaccurate and does not align with the facts. So there was no quid pro quo," said Mark Toner, deputy spokesman at the State Department.
In July, the FBI wrapped up its investigation of Mrs Clinton's use of her personal email system and concluded that no criminal charges would be appropriate.
FBI Director James Comey said the former top US diplomat had been "extremely careless" in her handling of classified information.
With the US election just three weeks away on 8 November, Mrs Clinton's Democratic campaign sought to downplay the new FBI documents.
"It's very well-known that there were disputes between the State Department and other agencies about classification," her campaign manager, Robby Mook, told reporters.
"It's not uncommon for officials within a department to fight over classification."
Melania defends Trump over lewd comments
Meanwhile, the wife of Donald Trump has said her husband was "egged on" while using lewd and sexually aggressive language while discussing women in a 2005 videotape that surfaced earlier this month.
In an interview with CNN, Melania Trump said she told her husband that the "language was inappropriate".
She continued: "It's not acceptable. And I was surprised, because that is not the man that I know," she said.
