The National Security Agency is under fire for new revelations that it used a “loophole” in federal law to search Americans’ calls and emails.

A program ostensibly aimed at foreigners also targeted information of “U.S. persons,” Director of National Intelligence James Clapper James Robert ClapperOn China, Biden is no Nixon — and no Trump The Hill's 12:30 Report - Speculation over Biden's running mate announcement Trump slams former intelligence officials to explain 'reluctance to embrace' agencies MORE said in a letter to Sen. Ron Wyden (D-Ore.), obtained by The Guardian on Tuesday.

Some searches used “U.S. person identifiers” to pick up “foreign intelligence targeting non-U.S. persons reasonably believed to be located outside the United States,” Clapper wrote.

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He did not specify how many times the searches included requests for information about Americans but said the NSA used “minimization procedures” to protect personal information and was "consistent with" the Fourth Amendment to the Constitution, which protects against unreasonable searches and seizures.

Clapper’s letter amounts to the first time the government has publicly confirmed the agency has searched for information about Americans’ calls and emails without a warrant.

Critics of the agency were incensed.

"This is unacceptable,” said Wyden and Sen. Mark Udall Mark Emery UdallThe 10 Senate seats most likely to flip Democratic presidential race comes into sharp focus Democrats will win back the Senate majority in 2020, all thanks to President Trump MORE (D-Colo.) in a joint statement.

“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," they added. "This fact should be beyond dispute.”

Sen. Richard Blumenthal (D-Conn.) added that the news “is further evidence that the intelligence community will spy on Americans if it believes it has the legal authority to do so, and therefore it must be reined in by stronger protections and oversight.”

Snooping programs under the legal authority disclosed on Tuesday are different from the NSA’s collection of records about millions of Americans’ phone calls, which are made up of metadata, not the content of people’s communications. President Obama has called for an end to that program, and the stage is set for Congress to take action.

The law with the legal "loophole" authorizes the PRISM program, which allows the agency to collect information from major tech companies like Google, Apple and Facebook, as well as UPSTREAM, which tapped into the network of undersea Internet fiber cables.

Lawmakers critical of the NSA’s programs have called for Congress to pass broad reforms to the spy agency’s programs and not just focus on the specific phone metadata proposal outlined by the White House.