The inspector general who oversees US intelligence agencies wrote in a letter to members of Congress on Thursday that a sampling of 40 of about 30,000 emails sent or received by Clinton found at least four that contained information the government had classified as secret.

The information was classified at the time the emails were sent, according to the inspector general, Charles McCullough. "This classified information should never have been transmitted through an unclassified personal system," McCullough said in a joint statement on Friday along with his equivalent at the State Department, Steve Linick.

The information remains classified to this day, the statement cited. The emails in question are not among those the State Department, which plans to eventually release as many as information laws allow, has already made public.

Clinton has said no classified information was contained in her emails, a large portion of which she handed over to the State Department last year. It remains unclear if the classified information was included in emails sent by Clinton, or only in those received by her. The inspectors general do not say whether they believe Clinton was aware the information was classified.

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