WASHINGTON — United States intelligence analysts have searched for Americans’ emails and phone calls within the repository of communications that the government collects without a warrant, according to a letter from James R. Clapper Jr., the director of national intelligence, to Senator Ron Wyden, Democrat of Oregon.

The March 28 letter was not the first official confirmation that both the National Security Agency and the C.I.A. had carried out such searches. But its release served to elevate attention to the fact that the activity, which Mr. Wyden has criticized as a “backdoor search” loophole to warrant requirements, was not just theoretical.

“It is now clear to the public that the list of ongoing intrusive surveillance practices by the N.S.A. includes not only bulk collection of Americans’ phone records, but also warrantless searches of the content of Americans’ personal communications,” Mr. Wyden said in a joint statement with Senator Mark Udall, Democrat of Colorado. “This is unacceptable. It raises serious constitutional questions, and poses a real threat to the privacy rights of law-abiding Americans.”

A 2008 law, the FISA Amendments Act, legalized the warrantless surveillance program that the Bush administration created after the terrorist attacks of Sept. 11, 2001. The law permits the government to intercept phone calls and emails without a warrant and on domestic soil, as long as the surveillance target is a noncitizen who lives abroad.