Washington (CNN) Democratic presidential candidate Bernie Sanders said Sunday that rival Hillary Clinton's email controversy is a "very serious issue," even though he has refused to attack the former secretary of state on that front.

Asked Sunday by CNN's Jake Tapper on "State of the Union" whether voters should interpret his refusal to engage Clinton on the email issue to mean that Clinton did nothing wrong, Sanders replied with a firm, "No."

"Nope, nope. That is not, I think, a fair assessment. That is, I think, a very serious issue," Sanders said. "There is a legal process taking place, I do not want to politicize that issue. It is not my style."

Sanders famously said in the first Democratic debate that he was tired of hearing about Clinton's "damn emails," brushing aside one of the biggest issues Republicans have raised to attack Clinton during the 2016 presidential cycle.

Sanders assessment that Clinton's use of private email on a private server during her time as secretary of state is a "very serious issue" comes after the State Department announced Friday that it was withholding 22 emails determined to be "top secret" by the intelligence community. The emails were not marked as classified at the time they were sent.

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