Marcel Lehel Lazar, better known as the Romanian hacker Guccifer, has claimed in an interview with NBC News that he didn't just publicly expose U.S. presidential candidate Hillary Clinton's email correspondence and address, but actually broke into her email server.

Clinton's camp denied this claim, noting that the FBI's past review of Clinton's server logs turned up zero evidence of infiltration, reported NBC. Lazar did not back up his statement with proof, but investigators nonetheless will question him, the report continued.

"It is unfathomable that he would have gained access to her emails and not leaked them the way he did to his other victims,” said Brian Fallon, national press secretary for Clinton's presidential campaign, in the article. Guccifer was recently extradited to the U.S. to face charges that he hacked various accounts belonging to prominent politicians and their families.

Will Donaldson, a former FBI network security expert witness and CEO of secure communications tool Nomx, told via email that the claims could have validity. "Guccifer has already proven that the unauthorized MX [mail exchange] server existed when he released the first emails...," said Donaldson, who was the first webmaster for the United States Marine Corps. "There is no reason to believe that he doesn't have the entire server, because he has already shown that he can exploit many types of systems."