Despite his insistence that Hillary Clinton’s private email server had been hacked, Donald Trump was unable to explain Thursday what evidence indicated that the server had been compromised.

In a preview of an interview set to air Thursday evening, NBC’s Lester Holt asked Trump what proof he’d seen that Clinton’s server had been hacked. Trump proceeded to argue that it was illegal for Clinton to have a private email server in the first place and lamented that she would not be charged because of a “rigged system.”

“You don’t know that it hasn’t been [hacked],” Trump said. “What she did is illegal. She shouldn’t have had a server.”

When Holt pressed Trump to say what evidence he’d seen of a hack, Trump struggled to respond.

“I think I read that and I heard it and somebody—” Trump said.

“Where?” Holt asked.

“Somebody gave me that information,” Trump said. “I will report back to you.”

The private email server has been a common talking point for conservatives and Trump, who brought it up several times Wednesday in an attack speech against Clinton.

Watch the clip below via NBC News: