FBI documents viewed in secure areas of the US Capitol

Lawmakers and select staff from some congressional committees could page through the material, parts of which are redacted, in large binders labeled 'secret' that the Federal Bureau of Investigation turned over to Congress on Tuesday.

Republican lawmakers requested the information last month after the FBI recommended no criminal charges against Clinton.

"Last I heard, my colleagues had their turn with the documents at 1:30 pm," said a Senate Judiciary committee aide. "There's only one set of documents for all the interested Senate committees, as I understand it."

She said those viewing the material were allowed to take notes. But if notes were taken on the classified parts, those notes had to be left in the secure area with the documents.

Clinton has been dogged for more than a year by questions about her use of a private email account while she was the nation's top diplomat from 2009 to 2013.

Republicans have repeatedly hammered Clinton over the issue, helping to drive opinion poll results showing that many US voters doubt her trustworthiness. She faces Republican nominee Donald Trump in the November 8 election.

The FBI said on Tuesday it had 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.

Senate Judiciary Committee Chairman Chuck Grassley, a Republican, said in a statement on Tuesday an initial review of the material showed most of it was marked unclassified.

Grassley, who has called for unclassified parts of the documents to be made public, wrote on Wednesday to the Senate Security Office, which has custody of the documents, asking them to provide his committee with an unclassified version.

Clinton campaign spokesman Brian Fallon in a statement also called for the public release of the documents, "rather than allow Republicans to mischaracterize them through selective, partisan leaks."

The documents on the Senate side of the Capitol are under the control of the Office of Senate Security, Senate aides said. In the House of Representatives, another set was being viewed in a "Secure Compartmented Information Facility," an aide with the House Oversight and Government Reform committee said.

"Our investigators with (security) clearances have gone down there and looked at the documents," the Oversight committee aide said.

It was unclear how many lawmakers had actually seen the material so far. Most members of Congress were not in town because of a congressional recess.

The documents were made available to the Judiciary, Homeland Security, and Appropriations committees in the Senate, and the Oversight, Judiciary and Appropriations committees in the House, aides said.

The Intelligence committees on both sides of the Capitol also have copies, so they do not have to join the line with the others.