In March, Clinton said she exchanged about 60,000 emails in her four years in the Obama administration, about half of which were personal and were discarded. She turned over the other half to the State Department in last December.

"Department employees circulated these emails on unclassified systems in 2009 and 2011 and ultimately some were forwarded to Secretary Clinton," said State Department spokesman John Kirby. "They were not marked as classified.".

Those two emails were among four that had previously been determined by the inspector general of the intelligence community to have been classified at the time they were sent. The State Department disputes that the emails were classified at that time.

Federal investigators have begun looking into the security of Clintons' email setup amid concerns from the inspector general for the intelligence community that classified information may have passed through the system.

The inspector general for the intelligence community had told Congress that potentially hundreds of classified emails are among the cache that Clinton provided to the State Department.

The department is reviewing those emails and has begun the process of releasing them to the public.

"As she has said, it is her hope that State and the other agencies involved in the review process will sort out as quickly as possible which emails are appropriate to release to the public, and that the release will be as transparent and timely as possible," Merrill said Tuesday.

Word that Clinton had relented on giving up possession of the server came as Republican Sen. Chuck Grassley of Iowa said two emails that traversed Clinton's personal system were deemed "Top Secret, Sensitive Compartmented Information"-- a rating that is among the government's highest classifications. Grassley said the inspector general of the nation's intelligence community had reported the new details about the higher classification to Congress on Tuesday.

Clinton had to this point refused demands from Republican critics to turn over the server to a third party, with attorney David Kendall telling the House committee investigating the 2012 attacks in Benghazi, Libya, that "there is no basis to support the proposed third-party review of the server.".

The decision advances the investigation into the Democratic presidential front-runner's use of a private email account as the nation's top diplomat, and whether classified information was improperly sent via and stored on the home-brew email server she ran from her house in suburban New York City.

It's not clear if the device will yield any information-- Clinton's attorney said in March that no emails from the main personal address she used while secretary of state still "reside on the server or on back-up systems associated with the server.".

Kendall gave the thumb drives, containing copies of roughly 30,000 emails, to the FBI after the agency determined he could not remain in possession of the classified information contained in some of the emails, according to a U.S. official briefed on the matter who was not authorized to speak publicly.

Earlier this week, Clinton said in a sworn statement submitted to a federal judge that she has turned over to the State Department all emails from the server "that were or potentially were federal records." The statement, which carries her signature and was signed under penalty of perjury, echoed months of Clinton's past public statements about the matter.

"Secretary Clinton's previous statements that she possessed no classified information were patently untrue," House Speaker John Boehner said in a statement. "Her mishandling of classified information must be fully investigated.".

Clinton has defended her use of the server, saying she used it as a matter of convenience to limit the number of electronic devices she had to carry.

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

Hillary Clinton relented Tuesday to months of demands she relinquish the personal email server she used while secretary of state, directing the device be given to the Justice Department.

Also Tuesday, Kendall gave to the Justice Department three thumb drives containing copies of work-related emails sent to and from her personal email addresses via her private server.

There is no evidence she used encryption to shield the emails or her personal server from foreign intelligence services or other potentially prying eyes. Kendall has said previously that Clinton is "actively cooperating" with the FBI inquiry.

Republicans jumped on Tuesday's decision to change course, as well as the additional disclosure that two emails that traversed Clinton's personal system were subsequently given one of the government's highest classification ratings.

Clinton campaign spokesman Nick Merrill said she has "pledged to cooperate with the government's security inquiry, and if there are more questions, we will continue to address them.".

"All this means is that Hillary Clinton, in the face of FBI scrutiny, has decided she has run out of options," Republican National Committee Chairman Reince Priebus in a statement. "She knows she did something wrong and has run out of ways to cover it up.".