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.