Hillary Clinton sent classified information via personal email

The internal review of Clinton's use of a personal account by the Inspector General for the intelligence community examined just 40 emails of the thousands sent through the account and found four of them contained information that should have been classified as "secret," the newspaper said. None of the emails were marked as such.

At the time they were sent, that was the second highest level of classification in the U.S. government.

Details of the review were included in a letter sent to members of Congress by the Inspector General for the Office of the Director of National Intelligence (ODNI). The Wall Street Journal said it had been provided a copy of the letter.

The finding has sparked a probe at the counterintelligence division of the FBI that will look into whether the Democratic presidential hopeful is guilty of mishandling classified information.

The controversy surrounds Clinton's use of a private email server and address while she was Secretary of State.

At the time, use of a government email address wasn't mandatory, according to the State Department, but it did mean that Clinton's emails could have escaped requests under the Freedom of Information Act.

The email should also not have been used for classified information. In fact, even state.gov email addresses aren't used for classified information. Government rules say classified data should never be sent via email but over one of several secured computer networks operated by the U.S. government specifically for such sensitive information.

Back in March, as she began ramping up her bid for the Democratic presidential nomination, Clinton handed over 55,000 printed pages of email to the State Department, and the inspector general's review has been focused on these.

Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is martyn_williams@idg.com