MUNICH, GERMANY - JUNE 23: Radomes at a facility called the Bad Aibling Station once used by U.S. National Security Agency (NSA) stand at dusk on June 23, 2014 near Bad Aibling, Germany. According to media reports based on recent documents released by former NSA worker Edward Snowden the NSA continues to operate from another nearby facility called the Mangfall Kaserne of the German intelligence services. The documents released by Snowden show a high level of activity of the NSA within Germany as well as active sharing of information between the NSA and German authorities. The Bundestag has convened a special commission to investigate the activities of the NSA following the revelation last year that the NSA had tapped the phone of German Chancellor Angela Merkel. (Photo by Joerg Koch/Getty Images)

The nation's top intelligence agency revealed in a letter made public Monday that the NSA, CIA and FBI are engaging in a large number of warrantless searches of the content of Americans' communications caught up in collection on foreign targets.

The letter, sent by the Office of the Director of National Intelligence to Sen. Ron Wyden (D-Ore.), also disclosed that there are large gaps in how the government tracks searches on Americans' communications -- and that the FBI simply does not track such searches at all.

Those revelations immediately raised an alarm for Wyden, who has begged the NSA for years to reveal how many "backdoor searches" it conducts, and on June 5 in Senate questioning pressed again for more detail.

"When the FBI says it conducts a substantial number of searches and it has no idea of what the number is, it shows how flawed this system is and the consequences of inadequate oversight," Wyden said in a statement. "This huge gap in oversight is a problem now, and will only grow as global communications systems become more interconnected."

The intelligence agencies for years have resisted making public the details of such searches, conducted under a 2008 law meant to bring President George W. Bush's warrantless wiretapping program under court review.

A provision of the law crafted then, Section 702 of the Foreign Intelligence Surveillance Act, was meant to ensure that only foreigners would be targeted for NSA collection. But the agency collects both sides of communications sent abroad, meaning that the agency picks up the content of Americans' communications "incidentally" collected. The Washington Post reported on Monday that the NSA considers 193 foreign countries -- containing the vast majority of the earth's population -- to be valid foreign intelligence targets.

"The Foreign Intelligence Surveillance Court has noted that the NSA acquires more than two hundred and fifty million Internet communications every year using Section 702, so even if US communications make up a small fraction of that total, the number of U.S. communications being collected is potentially quite large," said Wyden.

The Director of National Intelligence document disputes the suggestion made by critics like Wyden that the warrantless collection of Americans' communications violates the Fourth Amendment's prohibition on unreasonable government searches.

"Contrary to some claims, there is no loophole in the law, nor is the Intelligence Community conducting unlawful or 'backdoor searches' of communications of U.S. persons," the document states.

As evidence that the searches are not unlawful, the document points to a federal district court ruling last week in Oregon upholding the constitutionality of such searches.

Higher courts, however, have sidestepped deciding whether such searches are constitutional. In January 2013 the Supreme Court dodged the issue, ruling that human rights groups could not prove their communications were being caught up in collection under the 2008 law. The Snowden leaks later revealed that the government had failed to inform criminal defendants when such searches contributed to charges against them.

The Director of National Intelligence document states that the NSA approved 198 U.S. person identifiers -- essentially search terms associated with U.S. citizens, residents or organizations -- for queries of the contents of communications in its massive database of information collected under the 2008 law. Because the agency may define "persons" to include corporations or organizations, the number of human beings whose communications are read or listened to could be much greater.

The NSA made another 9,500 queries on metadata -- information about how communications were made and when, but not their actual content -- from the foreign intelligence database in 2013. That number is separate from, and far larger than, the 248 persons who had their metadata searched in databases like the controversial bulk domestic phone-records program.

While the NSA has been the focus of the post-Snowden surveillance debate, the letter also reveals that other agencies have had a far larger hand in warrantless surveillance than previously known. The CIA conducted "fewer than 1,900 queries" for the content of communications in the FISA database, and did not track the number of metadata queries it made.

The FBI did not track the number of queries it made for information on Americans, but the agency "believes the number of queries is substantial." The bureau did not immediately respond to a request for comment on the protections for Americans' privacy.

More information could be revealed Wednesday about the collection under the Foreign Intelligence Surveillance Act. The Privacy and Civil Liberties Oversight Board, an independent agency within the executive branch, is scheduled to release a report on the program.

The House voted earlier this month to stop those searches, and Wyden promised to press similar efforts in the Senate.