Months after saying Americans were "sick and tired of hearing about [Hillary Clinton's] damn emails," Bernie Sanders has conceded that "serious problems" exist in how Clinton handled classified material on her private email server as secretary of state.

"I think what the FBI report said is there were serious problems and that's what the case was," the Vermont senator told CNN's Wolf Blitzer during an interview Wednesday afternoon. "What we can be 100 percent assured of is the Republicans will politicize this issue and they will do everything they can to keep it in the papers."

"But Comey was very, very tough on her," Blitzer interjected, noting that House Speaker Paul Ryan has suggested that Congress strip Clinton of her security clearance.

"He was, alright," Sanders said. "And that's his job — to be objective, to be fair and to conduct I think what appears to be a serious investigation."

FBI Director James Comey on Tuesday declined to pursue criminal charges against Clinton, despite describing the presumptive Republican presidential nominee's handling of sensitive material as "extremely careless."