At least four emails from the private email account that former U.S. Secretary of State Hillary Clinton used while in office contained classified information, according to a government inspector’s letter that has deepened the email controversy dogging Clinton’s 2016 presidential campaign.

The inspector general who oversees U.S. 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 via an unclassified personal system,” McCullough said in a joint statement on Friday along with his equivalent at the State Department, Steve Linick.