US presidential candidate Hillary Clinton has been questioned by the FBI over her use of emails while she was secretary of state, her campaign says.A spokesperson said it was a voluntary interview.The FBI is investigating Mrs Clinton and her aides over whether they mishandled classified information on a private email server she used while serving as secretary of state. Mrs Clinton denies handling classified information in her private emails.She said she set up the email address for reasons of convenience, because it was easier to do everything from her Blackberry than to have several phones or tablets.However, a state department inquiry accused her and other former US secretaries of state of poorly managing email security.The justice department is now seeking to establish whether this constitutes a criminal offence.