Former New York City Mayor Rudy Giuliani is not pleased with Hillary Clinton, claiming she lied to the FBI during its investigation into her private email server.

The former secretary of state did actually know the "C" on an email meant "confidential," the Donald Trump surrogate said in an appearance on "Fox & Friends" Tuesday morning.

"She said she thought it had to do with alphabetical order. But the FBI didn't go and ask her, 'was there an A, a B, a D, an E, an F?' How come she just saw a C? So now if you can't figure out that 'C' doesn't mean confidential and you're the secretary of state, you're either stupid or you're lying. I don't think she's the first. I don't think she's stupid," Giuliani said.

Giuliani then rejected the idea that Clinton didn't know she was sending "classified" information because the "C" meant "confidential."

"The only person who can declassify is the president of the United States. Now maybe she thought she was the president. Maybe she was having some kind of illusion," Giuliani said, before adding, "I don't think she was having an illusion. I think she is doing something called lying which violates 18 United States Code 1001."

Last week, the FBI released a report on its investigation into Clinton's use of a private email server. In the report, she told federal agents that she did not believe she received any emails that were too secretive for her private email server to handle.

The FBI report also shows no evidence that Clinton's emails were hacked or that she broke the law.

Giuliani remarked that Clinton's emails show criminal intent.

"As a prosecutor, I don't think I ever had a case with stronger evidence of criminal intent than that," he said.