The former head of U.S. intelligence said on Sunday that homegrown terrorism was “a devil of a problem” and the most serious threat facing American citizens.

“In a democracy it’s incredibly difficult,” former CIA Director Michael Hayden told Candy Crowley on CNN’s “State of the Union.” “Look, we’ve all made our compromises with al Qaeda and the al Qaeda kinds of attacks.”

“But how do you build a security structure that guards you against American citizens who are beginning to change in their thinking up to a point where they become a threat to the security of other Americans? That’s a devil of a problem.”

Hayden went on to point out that the next step the intelligence community would take to combat homegrown terrorists would be to infringe on the privacy of Americans, and that was still too steep of a price to pay.

“What are you or your viewers willing to pay?” he asked Crowley. “How much would you allow us commerce or privacy or convenience in order to get down to that level of granularity? And frankly, I think American political culture — I think you and I, as citizens, would be uncomfortable going very far in that direction. That what makes this such a devilish problem.”

Hayden also emphasized that the U.S. military should stay in Afghanistan and that al Qaeda’s influence in the country was waning becomes of the presence of U.S. armed forces. If the U.S. withdrew prematurely, he said, it would be detrimental to American security.

“I would let this go for a while longer,” he said. “With regards to the small number of al Qaeda in Afghanistan, that may be a reflection of American combat power in Afghanistan and if one were to remove that combat power, one would naturally see the number of al Qaeda rise.”