The US State Department has restarted its internal investigation into Hillary Clinton's use of a private email server while she was Secretary of State.

The presumptive Democratic candidate has repeatedly said that no email she sent or received was marked classified.

An FBI investigation into the controversy concluded that "no charges are appropriate".

Confirming an internal investigation, State Department spokesperson John Kirby said: "We will aim to be as expeditious as possible, but we will not put artificial deadlines on the process."

A number of Mrs Clinton's former aides are also likely to be investigated, BBC reports, and administrative sanctions could still apply depending on the investigation's result.

According to The Associated Press, the investigation was opened in January after they declared 22 emails from Mrs Clinton's private server to be "top secret". However, it was suspended to allow for the FBI investigation.

Outlining his agency's findings at a news conference, FBI director James Comey concluded that "no charges are appropriate" despite "extremely careless" behaviour by Mrs Clinton and her staff.

"Although there is evidence of potential violations of the statutes regarding the handling of classified information, our judgement is that no reasonable prosecutor would bring such a case," he said.

The FBI announcement came several days after agents interviewed Mrs Clinton about the possible mishandling of classified information.

The email server controversy has been seized on by Republicans to criticise Mrs Clinton ahead of the November election, with Donald Trump tweeting that the "rigged system may have helped Hillary Clinton escape criminal charges".