The controversy surrounding former US Secretary of State Hillary Clinton's email that allegedly contains hundreds of classified information has reached a new level after some intelligence experts called for a criminal inquiry on the Democratic presidential frontrunner.

Kevin Carroll, a former officer from Central Information Agency and a secrecy lawyer in Washington, questioned the Justice Department's failure to open a criminal investigation against Clinton.

Carroll expressed concern that Clinton might have compromised hundreds of spy agency secrets that were found in her emails, which, he said, could be worse that what CIA Director David Petreaus did in 2011 when the latter was found to have mishandled classified information for which he was sentenced to two years probation and a $100,000 fine.

"I don't see how the Justice Department would be able to avoid at least investigating this. What Petraeus did was really small in comparison, because there was no exposure of any information to any foreign intelligence services," Carroll said, according to Washington Times.

"Information put on her home-cooked server and then sent around to other accounts is a very, very serious counterintelligence breach. They're going to have a really substantial look at the damage that's been done to every agency that's had its intelligence compromised," he added.

Petreaus admitted to mishandling classified military files to his brother and mistress during his term as the American military commander in Afghanistan four years ago, prompting him to resign in 2012.

It is against the law to remove classified information from government facilities and retain it after you have left office and have no official reason to possess it, the Washington Post said in an article.

The former CIA officer's mishandling of classified military information, according to Carroll, paled in comparison to what Clinton has done, adding that her email must have been accessed by foreign intelligence services.

Reports earlier said that Clinton was also caught removing at least five e-mails containing classified information and retaining them on her personal server in her home in Chappaqua, New York, after she left office.

Last July 27, two inspectors-general asked the Justice Department to open an investigation into whether sensitive government information was mishandled in connection with the personal email that Clinton used as secretary of state.

But according to the New York Times, the Justice Department remains uncertain on whether it will open an investigation against Clinton.