Syndicated columnist Charles Krauthammer said on Fox News' “Special Report with Bret Baier” that there are "a lot of mysteries" surrounding Gen. Michael Flynn's resignation.

Where Flynn erred was lying to Vice President Mike Pence about speaking with the Russian ambassador, Krauthammer argued, it was not the actual conversation itself that was the problem.

“It’s because he lied about something material to the vice president of the United States—left him hanging, telling a lie in public. You cannot do that,” he said. “Now I’ve maintained from the beginning this is a cover up without a crime. There is nothing wrong with an incoming national security adviser talking with the Russians, talking about sanctions with the Russians. It’s not illegal, it’s not improper. But you can’t lie to the vice president about it.”

But Krauthammer said an investigation is needed because there are “a lot of mysteries” surrounding Flynn—one of which is why he would have lied.

“It was clearly an innocent act in the first place, is there anything else he’s covering up? Are there any other contact he might have been afraid would come out?”

Check out the full clip below. The relevant section with Krauthammer begins around the 5:15 mark.