Democratic presidential candidate Sen. Bernie Sanders (I-VT) called Republican presidential candidate front runner Donald Trump a “pathological liar” on Sunday.

Trump had earlier in Sunday news show appearances called Sanders a liar. The Vermont Senator responded to Trump on ABC News’ “This Week.”

“I tell you, it really is rather extraordinary,” Sanders replied. “I think — and I say this straightforwardly — I think you have a pathological liar there.”

Sanders pointed to Trump insisting that he saw people in New Jersey cheering on 9/11. Multiple fact checkers have dubbed this false.

“Nobody has seen a tape of thousands of people celebrating the destruction of the Twin Towers in New Jersey,” Sanders said. “It doesn’t exist. And he keeps claiming it. That’s called pathological lying.”