Sen. Bernie Sanders (I., Vt.) said he considered Hillary Clinton's private email server investigation to be "a very serious issue" Sunday on CNN, while also adding there was a legal process taking place that he did not want to politicize.

At the conclusion of their interview, State of the Union host Jake Tapper reminded Sanders of his famous remarks at the first Democratic debate in October, when he told Clinton the American people were "sick and tired of hearing about your damn emails."

On Friday, the Obama administration announced that 22 more emails from her private server would not be released because they had been classified as top secret.

"Should voters take from those comments that you think nothing was done wrong when it comes to how Secretary Clinton handled classified information, or is that not a fair–?" Tapper asked, before Sanders cut in.

"No, no," Sanders said, wagging his finger. "No, that is not, I think, a fair assessment. I think this is a very serious issue. I think there is a legal process right now taking place … I get criticized. ‘Bernie, why don't you attack Hillary Clinton?' There is a legal process taking place. I do not want to politicize that issue. It is not my style."

Sanders said he was focusing on the issues affecting the middle class rather than Clinton.

"Jake, I go around this state of Iowa. I can't go to a meeting where … middle-aged people [don't] tell me how how much in debt they are in terms of student debt or people who can't afford health care," Sanders said. "Those are the issues that we're going to focus on. There is a legal process underway with regards to the email situation. It will play out. I'm not going to politicize it."

A Clinton campaign strategist accused Sanders on Friday of running the "most negative" Democratic primary campaign ever. While Sanders has attacked Clinton for her Wall Street ties, he has avoided rhetoric about her private email scandal or Bill Clinton's past sexual misconduct coming back into the limelight.