Image copyright Reuters Image caption Critics say Mrs Clinton's use of a private email system was contrary to government policy

Hillary Clinton has agreed to hand over to the FBI the private email server that she used as secretary of state.

Her use of private email has generated a barrage of criticism as Mrs Clinton runs for president.

Critics say that her set-up was unsecure, contrary to government policy and designed to shield her communications from oversight.

The FBI is investigating whether classified information was improperly sent via the server and stored there.

Mrs Clinton initially handed over thousands of pages of emails to the state department, but not the server.

Her lawyers will also hand over to the FBI memory sticks which contain the copies of the emails.

Her use of private email has been a major issue in the presidential race. Polls show an increasing number of voters view her as "untrustworthy" due in part to the questions surrounding her email use.

Under US federal law, officials' correspondence is considered to be US government property.

'Emailgate' at a glance

Why did she do it?

Mrs Clinton says the primary reason she set up her own email was for "convenience" but sceptics say the real reason she did it was because it gave her total control over her correspondence.

How many emails?

According to Mrs Clinton, she sent or received 62,320 emails during her time as secretary of state - she says half of them were official and have been turned over to the State Department.

Was it illegal?

Probably not. Mrs Clinton's email system existed in a grey area of the law - and one that has been changed several times since she left office.

Why the controversy?

It's a big deal because Mrs Clinton is asking the US public to trust that she is complying with both the "letter and the spirit of the rules". Her "convenience" explanation has been difficult for some to swallow and critics on both the left and the right are concerned she made her communications on sensitive national security issues more susceptible to hackers and foreign intelligence services.

For more, read Hillary Clinton's 'emailgate' diced and sliced

Government employees are encouraged to use government email accounts although some top officials have used personal accounts in the past.

In March, Mrs Clinton said she and her lawyers made the decision over what would be considered work-related email when the state department asked for records from former secretaries of state.

The emails deemed work-related were about half of the 60,000 emails she sent in total during her time in office. The emails she deemed personal were deleted, Mrs Clinton said.

Since then, the state department has been releasing the emails to the public in batches about once a month.

However on Tuesday, the state department said it would not be releasing a portion of the emails because some of the messages were retroactively determined to be "top secret".

Senior Republican Reince Priebus said Mrs Clinton had "run out of options" in the face of FBI scrutiny.

"She knows she did something wrong and has run out of ways to cover it up," he said.