Hillary Clinton was interviewed by FBI officers for three and a half hours on Saturday morning as part of an investigation into her use of a private email server when she was Secretary of State, a probe that could yet thwart her campaign to become America's first female president.

The FBI is investigating whether she broke any laws about the handling of classified information by using a server kept in her home in upstate New York.

Although she has apologised, saying she should have used it only for personal communications, critics have repeatedly seized on the issue, saying it shows she cannot be trusted with high office.