McCaul accused Clinton on Fox and Friends of taking devices abroad that contained classified e-mails even though the State Department advised against it.
"Now we find out –as James Comey told me previously–it's very likely that foreign adversaries from other nations got into her private server. This is why you have security protocols to protect classified information," McCaul said. "She exposed it to our enemies and now ... our adversaries have this very sensitive information that not only jeopardizes her and national security at home, but the men and women serving overseas. This is in my opinion quite frankly it's treason."
Fox News reported Thursday morning that according to two anonymous sources with knowledge of the FBI's investigation, the FBI was "99 percent" certain that foreign adversaries had been able to access emails on Clinton's private server.
The FBI said publicly in July while it was "possible" Clinton's email server had been hacked by foreign entities when she was abroad, there was no direct evidence it had happened.
McCaul's "treason" comment came just hours after he told Fox News Wednesday that Clinton should be "impeached" if she were elected.
"This investigation will continue whether she wins or not, but assuming she wins and the investigation goes forward, and it looks like an indictment is pending, at that point of time in the Constitution, the House of Representatives would engage in an impeachment trial," McCaul said Wednesday in reference to a congressional probes into Clinton's e-mails. "It would go to the Senate, and impeachment proceedings and removal would take place."