The National Security Agency has searched the communications of American citizens without obtaining warrants, the administration's top intelligence official confirmed this week.

James Clapper, director of National Intelligence, confirmed the practice in a letter to Congress Tuesday. He didn't say when or how often the searches took place.

The NSA gathers the communications of ordinary Americans, but it had not been clear if those records had been searched.

"Senior officials have sometimes suggested that government agencies do not deliberately read Americans' emails, monitor their online activity or listen to their phone calls without a warrant," Sens. Ron Wyden, D-Ore., and Mark Udall, D-Colo., said in a joint statement. "However, the facts show that those suggestions were misleading, and that intelligence agencies have indeed conducted warrantless searches for Americans' communications."

The Obama administration defended the practice because it says the communications were gathered legally.

Civil rights groups say collecting and searching the information violates the Constitution.