AP

WASHINGTON (AP) — The Obama administration has conducted warrantless searches of Americans' communications as part of the National Security Agency's surveillance operations that target foreigners located outside of the U.S., the administration's top intelligence official confirmed in a letter to Congress disclosed Tuesday.

These searches were authorized by a secret surveillance court in 2011, but it was unclear until Tuesday whether any such searches on Americans had been conducted.

The recent acknowledgement of warrantless searches on Americans offers more insight into U.S. government surveillance operations put in place after the terror attacks of Sept. 11, 2001. The government has broadly interpreted these laws to allow for the collection of communications of innocent Americans, practices the Obama administration maintains are legal. But President Obama has promised to review some of these programs to determine whether the government should be conducting this type of surveillance at all.

"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," Democratic Sens. Ron Wyden and Mark Udall 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."

Wyden has pressed the administration on whether these searches on Americans have occurred. In a March 28 letter to Wyden, James Clapper, the government's top intelligence official, said the NSA has searched for Americans' communications within information it collected when it targeted foreigners located outside the U.S. In his letter, Clapper also pointed to a declassified document released last August that also acknowledged the use of such searches and stated that these searches were reviewed, and there was no finding of wrongdoing. It was unclear how often these searches are conducted.

Documents disclosed last year by former NSA systems analyst Edward Snowden showed that the government collects mass amounts of data from major Internet companies such as Google, Apple, Microsoft and Facebook through one of its programs designed to target communications of foreigners located outside the U.S. The government is not allowed to use this authority to collect Americans' communications, but conversations of innocent Americans are collected inadvertently. When this happens, the NSA is required to take certain measures to hide the communications of Americans that have nothing to do with foreign intelligence.

In 2011, the government sought and received approval from the Foreign Intelligence Surveillance Court to search for Americans within the communications it already possessed through its collection of conversations of foreigners outside the U.S. Such searches would only be permissible if there were a foreign intelligence purpose.

Former NSA deputy director Chris Inglis said this authority might be used to search for the target of a terrorist attack. As an example, Inglis said if the government was concerned that terrorists were plotting to attack the New York Stock Exchange, the NSA could search for the term "New York Stock Exchange" among the conversations it collected in its targeting of foreigners overseas.

Wyden, Udall and other civil liberties advocates call this type of search a back-door loophole in the law that governs surveillance of Americans.

"If a government agency thinks that a particular American is engaged in terrorism or espionage, the Fourth Amendment requires that the government secure a warrant or emergency authorization before monitoring his or her communications," Wyden and Udall said.

The Obama administration contends the searches are legal because they are searching information they lawfully obtained.