The National Security Agency (NSA) is deleting years' worth of call records collected for foreign intelligence purposes, saying that “technical irregularities” resulted in the spy agency collecting data it was not authorized to receive.

The NSA issued a statement Thursday revealing that the spy agency started deleting all so-called call detail records (CDRs) collected since 2015 in May of this year.

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CDRs are obtained from telecommunications providers and contain the numbers and time and duration of phone calls, not the content of the calls themselves. The NSA is authorized to collect the data under the Foreign Intelligence Surveillance Act.

“NSA is deleting the CDRs because several months ago NSA analysts noted technical irregularities in some data received from telecommunications service providers,” NSA said. “These irregularities also resulted in the production to NSA of some CDRs that NSA was not authorized to receive.”

The revelation comes after an annual transparency report issued by the director of national intelligence (DNI) showed that the NSA collected well over 500 million U.S. call detail records in 2017 — more than three times the number gathered the previous year.

The NSA said that it is deleting all CDRs gathered since 2015 because it was “infeasible” for the agency to isolate the data it was authorized to receive from the rest of it. The NSA said it notified relevant oversight committees in Congress, as well as the Department of Justice and the independent Privacy and Civil Liberties Oversight Board, of the decision.

“The root cause of the problem has since been addressed for future CDR acquisitions, and NSA has reviewed and revalidated its intelligence reporting to ensure that the reports were based on properly received CDRs,” the NSA said.

The transparency report issued in early May showed that the NSA collected 534 million U.S. call records in 2017, well over the 151 million received in 2016. At the time, a DNI spokesman said the agency expects the figure to “fluctuate from year to year” and that a number of factors could influence the number received — including the number of court-approved selection terms used by a given target and “the dynamics of the ever-changing telecommunications sector.”

The NSA’s call collection efforts have been enveloped in controversy since the 2013 Edward Snowden revelations of massive global surveillance by the agency.

As a result of Snowden's disclosures, Congress passed legislation meant to rein in the surveillance program, which included a mandate that the intelligence community provide an annual report on its efforts in order to boost transparency.

Late last month, two-dozen civil liberties organizations wrote to Director of National Intelligence Dan Coats Daniel (Dan) Ray CoatsFBI chief says Russia is trying to interfere in election to undermine Biden The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Former Intel chief had 'deep suspicions' that Putin 'had something on Trump': book MORE urging him to disclose more information about the U.S. call records collected in 2017.