Washington (AFP) - The FBI has unexpectedly released documents concerning ex-president Bill Clinton's pardon of the husband of a wealthy Democratic donor, in a surprise move just days before the election in which his wife is seeking to become America's first female president.

The release of the heavily redacted 129-page report over the pardon of trader Marc Rich -- an investigation that closed in 2005 without charges -- triggered questions from Democrats already angered by the FBI's probe into hundreds of thousands of newly uncovered emails possibly linked to Hillary Clinton.

While the Rich documents were published online Monday, they received little notice until they were posted on Tuesday on a Twitter account for the Federal Bureau of Investigation's division managing Freedom of Information Act (FOIA) requests that had had no posts since a year ago, except for a small handful released simultaneously on Sunday.

"Absent a FOIA litigation deadline, this is odd," said Hillary Clinton spokesman Brian Fallon.

"Will FBI be posting docs on Trump's housing discrimination in '70s?" he added, referring to Clinton's Republican rival Donald Trump, a billionaire real estate magnate.

The FBI said the documents were posted shortly after they were processed, as with FOIA materials requested three or more times.

"Per the standard procedure for FOIA, these materials became available for release and were posted automatically and electronically to the FBI's public reading room in accordance with the law and established procedures," the statement said.

The FBI indicated that this was only a "preliminary" release that could therefore be followed by more.

Rich was indicted on federal charges of tax evasion in the United States. He was a fugitive from the Department of Justice -- at a time one of the FBI's most wanted -- living in exile in Switzerland at the time of his indictment. He died there in 2013.

In a controversial move, Bill Clinton pardoned him on his last day in office on January 20, 2001. The FBI opened its investigation into the pardon later that year.

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Rich's ex-wife Denise Eisenberg Rich, whose name was redacted from the FBI files, "has been a major political donor to the Democratic Party, and these donations may have been intended to influence the fugitive's pardon," reads a bureau note requesting that a preliminary investigation be opened.

Some of the donations went to the William J. Clinton Presidential Foundation, the predecessor to the Clinton Foundation, according to the document.

"It appears that the required pardon standards and procedures were not followed," reads the FBI document dated February 15, 2001.

The Rich case fell under the watch of current FBI Director James Comey, then a younger prosecutor.

The FBI document dump comes as Comey is under fire, from both Democrats and some Republicans, for effectively reopening in recent days the bureau's investigation into Hillary Clinton's use of a private email server.