Director of National Intelligence James R. Clapper Jr., acknowledged that the National Security Agency, or NSA, had conducted searches among Americans' communications without a warrant, in the first such admission of the practice from a senior intelligence official, according to The Washington Post, as senior members of the U.S. government, in a letter to Congress released Tuesday, accused the Obama administration and the NSA of spying on private communications of American citizens.

U.S. Senators Ron Wyden (D-Ore.) and Mark Udall (D-Colo.), who serve on the U.S. Senate Select Committee on Intelligence, said in a statement that the NSA exploited loopholes in the system to track the content of citizens' private communications, an activity that usually requires a warrant. While these searches were authorized in 2011 by a secret surveillance court after the 9/11 terrorist attacks, it was not clear until now if such searches were in fact conducted by the agency.

"It is now clear to the public that the list of ongoing intrusive surveillance practices by the NSA includes not only bulk collection of Americans' phone records, but also warrantless searches of the content of Americans' personal communications," the senators said in a joint statement, adding: "It raises serious constitutional questions, and poses a real threat to the privacy rights of law-abiding Americans.”

The Foreign Intelligence Surveillance Act, or FISA, established a legal framework under which the government could acquire intelligence by tracking non-U.S. citizens believed to be residing outside the country. However, during the process, if communications by Americans not related to foreign intelligence is inadvertently collected, the NSA is required to hide such information.

The senators accused the NSA and President Barack Obama's administration of exploiting the loophole of “back-door search,” according to section 702 of FISA, which allows spying on non-Americans without individual warrants. 

"The revelation that — despite the clear intent of Section 702 to target foreign communications — the government is deliberating searching for the phone calls or emails of specific Americans and circumventing traditional warrant protections should be concerning to all," Wyden and Udall said in the statement.

On Friday, Clapper had reportedly told Wyden in a letter that the NSA searched for communications made by Americans through the information it collected when it targeted foreigners located outside the U.S. In the letter, Clapper also referred to a document that was declassified last August that acknowledged such searches, reviewed them and found no wrongdoings, Associated Press reported.

Edward Snowden, the NSA whistleblower, had released documents last year that showed the government collected large amounts of data from Internet companies such as Google, Apple, Microsoft and Facebook through one of its programs designed to target communications of foreigners outside the U.S. The Obama administration has argued that these searches are legal as the information is obtained lawfully.

“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. However, the facts show that those suggestions were misleading, and that intelligence agencies have indeed conducted warrantless searches for Americans’ communications using the 'back-door search' loophole in section 702 of the Foreign Intelligence Surveillance Act,” Wyden and Udall added in the statement.