Democratic US presidential candidate Hillary Clinton speaks at the US Conference of Mayors 84th Annual Meeting in Indianapolis, Indiana, US, June 26, 2016. REUTERS/Chris Bergin The FBI interviewed Hillary Clinton Saturday morning in connection with the ongoing investigation into her use of a private email server as secretary of state, the presumptive Democratic presidential nominee's campaign said.

"Secretary Clinton gave a voluntary interview this morning about her email arrangements while she was secretary," spokesman Nick Merrill said.

He added: "She is pleased to have had the opportunity to assist the Department of Justice in bringing this review to a conclusion. Out of respect for the investigative process, she will not comment further on her interview."

The interview occurred at FBI headquarters in Washington, DC, and lasted approximately three and a half hours, a Clinton aide said.

The FBI's interview came more than a year after Clinton first admitted that she conducted official government business using a personal email address instead of one issued by the government.

Federal authorities began their investigation in August.

The FBI has been looking into whether classified material was mishandled during Clinton's tenure at the State Department from 2009 to 2013. Reports indicate classified information traversed her home server, but Clinton has maintained that such messages were not classified at the time.

Clinton, who is weeks away from officially accepting the Democratic Party's nomination for president at the party's July convention, has maintained that using the private email server was not illegal.

An inspector general's report released in May appeared to back up Clinton's claim, but said she "did not comply" with State Department policies requiring her to surrender "all emails dealing with department business before leaving government service."

Clinton has turned over tens of thousands of her work-related emails over the course of the investigation. About 30,000 more emails that were deemed "personal" were deleted.