Director of National Intelligence James Clapper confirmed the surveillance in a letter responding to questioning from Sen. Ron Wyden, an Oregon Democrat. The agency spied on the actual contents of communications without a warrant—not just “metadata” such as call times and phone numbers.

“This is unacceptable. It raises serious constitutional questions, and poses a real threat to the privacy rights of law-abiding Americans,” Wyden and Sen. Mark Udall, a Colorado Democrat, said in a statement…

Section 702 of the Foreign Intelligence Surveillance Act gives the NSA broad power to listen in on phone calls and access emails. But the law covers only non-Americans located outside of the United States.

The agency sometimes collects Americans’ information as it scoops up vast amounts of data on foreigners. In his letter to Wyden, Clapper revealed that the NSA has searched through that database specifically looking for Americans’ communications.