Hillary Clinton
Hillary ClintonReuters

The FBI has started investigating the security of the controversial private email setup used by Democratic presidential candidate Hillary Clinton back when she was US Secretary of State.

Two government officials revealed to the Washington Post that the FBI last week got in contact with the Denver-based technology company that managed Clinton's system, and likewise contacted Clinton's lawyer, David Kendall, about the security of a USB drive he holds, which contains copies of Clinton's work emails.

Clinton is facing criticism for having used a personal email address during her entire four years as secretary of state, despite the fact that under specific guidelines signed into law by President Barack Obama, government email accounts are supposed to be used in any situation involving official business so as to enable accountability and transparency.

Clinton late last month denied sending classified emails with the personal account, just after it was revealed that an internal government review had found that Clinton sent at least four emails from her personal account containing classified information.

Clinton campaign spokesperson Nick Merrill reiterated her denial with a caveat to Fox News on Tuesday, saying that "she did not send nor receive any emails that were marked classified at the time."

By noting the emails weren't classified "at the time," Merrill contradicted Clinton's claim to reporters in March that no classified material was ever sent through her account.

"I did not e-mail any classified material to anyone on my e-mail. There is no classified material. I'm certainly well-aware of the classification requirements and did not send classified material," Clinton said at the time.