Facing questions: Hillary Clinton at a campaign stop in Claremont, New Hampshire, Tuesday

Hillary Clinton will turn over her email server to federal investigators - after it emerged that the insecure device contained at least two hyper-classified messages.

The former Secretary of State agreed Tuesday to give her controversial server to the Department of Justice.

The move came on the same day the chairman of Congress's judicial committee revealed that some of the messages on the private server were stored at the government's second-highest level of secrecy.

Chuck Grassley (R-Iowa) revealed that a government inspector found messages on Clinton's server marked 'Top Secret, Sensitive Compartmented Information'.

A spokesman for Clinton's presidential campaign said the candidate 'pledged to cooperate with the government's security inquiry' and answer further questions.

Two thumb drives containing a cache of her emails were also handed over to FBI agents Tuesday after officials deemed they could no longer safely be held in the Washington, D.C., officer of her lawyer, where they had been kept.

The two super-secret emails - plucked from a sample of just 40 from Clinton's tens of thousands of messages - are thought to originate from the CIA.

The Washington Post reported that the agency had originated the information, and said it could be linked to satellite imagery.

Critics of Clinton in Congress, government and beyond have called for months for her to release more information about her emails - but the former secretary has proved reluctant.

In light of the handover, and the federal investigation against her, leading Republicans renewed their attacks on Clinton, accusing her of attempting a cover-up by sending the information to the government of which she used to be a part - rather than an independent arbiter.

Reince Priebus, chairman of the Republican National Committee, said: 'If Hillary Clinton believed in honesty and transparency, she would have turned over her secret server months ago to an independent arbiter, not as a last resort and to the Obama Justice Department.

'Of course, if she really cares about transparency, she would never have had a secret server in the first place.

'All this means is that Hillary Clinton, in the face of FBI scrutiny, has decided she has run out of options. She knows she did something wrong and has run out of ways to cover it up.

Hillary Clinton on the campaign trail in New Hampshire Tuesday before the news broke. Her team says she is handing over her private server to the Department of Justice

Clinton used to run her private email setup on an insecure server in her home in Chappaqua, NY (above)

'The majority of Americans don't trust her, and they're right. She put our national security at risk for selfish reasons. She couldn't be trusted at the State Department - she certainly can't be trusted in the Oval Office.'

House Speaker John Boehner also weighed in, accusing Clinton of lying earlier about having had no classified information on the servers.

He said: 'It's about time. Secretary Clinton's previous statements that she possessed no classified information were patently untrue. Her mishandling of classified information must be fully investigated.'

Meanwhile representative Trey Gowdy (R-South Carolina), who chairs the Select Committee on Benghazi, said that the revelations should 'concern' people everywhere.

He said: 'The IC Inspector General revealed Secretary Clinton’s emails and server contained not just "top secret" classified information, but "compartmented" classified intelligence not releasable to foreigners, which must be noted in the timing of this announcement.

'Both the ICIG and the State Department IG were nominated by President Barack Obama and confirmed by the Senate when it was controlled by Democrats.

'Secretary Clinton's decision to prioritize her own convenience - and desire for control - over the security of our country's intelligence should concern all people of good conscience

'This is a serious national security issue, and the seriousness of it should transcend normal, partisan politics.'

The State Department previously had said it was comfortable with Kendall keeping the emails at his Washington law office.

The department also disputes that the emails were classified at the time they were sent.

The U.S. official said the FBI recovered at least two thumb drives containing the emails from Kendall.

The drives contain around 30,000 emails that Clinton deemed work-related and turned over to the State Department.

She destroyed thousands of others that she said were not work-related.

McCullough told Congress that potentially hundreds of classified emails are among the cache that Clinton provided.

The Clinton presidential campaign and Kendall did not immediately respond to a request for comment.

The FBI is looking into the security of the Clinton email arrangement. There is no evidence she used encryption to shield the emails from foreign intelligence services or other prying eyes.

In response to the allegations, a spokesman for Hillary Rodham Clinton's presidential campaign has said that Clinton asked for the personal email server to be given to the Justice Department.