By Susan Cornwell

WASHINGTON (Reuters) - The FBI said on Tuesday it has turned over to the U.S. Congress a number of documents related to its investigation of Hillary Clinton's use of a private email server while she was secretary of state.

The Democratic presidential nominee has for over a year been dogged by questions about her use of a private email account while she was the nation's top diplomat.

Republicans have repeatedly hammered Clinton over the issue, helping to drive consistent opinion poll results showing that some U.S. voters doubt her trustworthiness.

The Federal Bureau of Investigation said it provided "relevant materials" to congressional committees looking into the matter.

"The material contains classified and other sensitive information and is being provided with the expectation it will not be disseminated or disclosed without FBI concurrence," the agency said in a statement.

A spokeswoman for the House of Representatives Oversight and Government Reform Committee said in an email that staff for the panel were reviewing the information classified SECRET.

"There are no further details at this time," the aide said.

FBI Director James Comey told Congress last month that Clinton's handling of classified information while using private email servers was "extremely careless." But he said he would not recommend criminal charges be brought against her.

Comey's statement lifted a cloud of uncertainty from Clinton's White House campaign. But his strong criticism of her judgment ignited a new attack on her by Republicans, including Donald Trump, her Republican opponent in the Nov. 8 election.

The Oversight committee, chaired by Representative Jason Chaffetz, had asked the FBI for the complete investigative file from its review of Clinton's use of a private email server.

The FBI also provided documents from its investigation to the House Judiciary Committee, an aide said.

Chaffetz and Judiciary Committee Chairman Bob Goodlatte, also a Republican, last month called for federal prosecutors to investigate whether Clinton had committed perjury.

They said some FBI findings about her email servers were at odds with her previous testimony to Congress about the matter, for example, her statement that she had not sent or received information designated as classified.

But the FBI, in a letter sent to the committee to accompany its investigative documents on Tuesday, said the fact that the FBI had uncovered three instances where Clinton received emails containing "(C)" markings for classified was "not clear evidence of knowledge or intent" to mishandle classified information.

The letter noted relevant emails had been forwarded to Clinton by staff, lacked "header and footer markings" indicating the presence of classified information, and only one email was later determined by the State Department to contain classified information.

Clinton's fellow Democrats were scornful that Republicans were refusing to let the matter drop.

"The FBI already determined unanimously that there is insufficient evidence of criminal wrongdoing. Republicans are now investigating the investigator in a desperate attempt to resuscitate this issue, keep it in the headlines, and distract from Donald Trump’s sagging poll numbers," said Representative Elijah Cummings, the top Democrat on the Oversight committee.

Representative Adam Schiff, the top Democrat on the House Intelligence Committee, said he could see little "legitimate purpose" to which Congress would put the FBI materials, predicting that they would be leaked for political purposes.

(Reporting by Susan Cornwell; Editing by Kevin Drawbaugh and Jonathan Oatis)