Two-dozen civil liberties organizations are urging U.S. officials to disclose more details on the more than 500 million call records collected on Americans by the National Security Agency (NSA) last year.

The organizations, which include the American Civil Liberties Union and digital rights group Access Now, say that the information is crucial to determining whether the government is overstepping its authority.

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The letters, sent Thursday to the Office of Director of National Intelligence and to the House Judiciary Committee, come following the publication of a transparency report that revealed the spy agency collected 534 million call detail records in 2017, a substantial increase over the previous year.

These records are obtained from U.S. telecommunications providers and include the number and time and duration of phone contacts, not the content of the calls themselves.

The civil liberties groups are particularly interested in the number of so-called “unique identifiers,” or unique accounts, devices, or individuals, swept up in the NSA’s call detail record program.

The annual report is mandated by the USA Freedom Act passed by Congress in 2015 that aimed to place limits over the spy agency’s surveillance program, following the Edward Snowden disclosures. While the law directs the intelligence community to report the number of unique identifiers, officials have said that they do not possess the technical ability to do so.

“Obtaining this data is particularly important given that the number of call detail records collected under Section 215 has surged to over 540 million in 2017 — more than triple what was reported for 2016,” the ACLU and other groups 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.

“Because the NSA has failed to report the number of unique identifiers impacted, we are left with no way to assess whether this surge is due to duplication, over-reporting, or an abuse of the government’s authority,” they wrote. “We are also concerned that the NSA’s failure to provide this information is further indicative of disregard of Congress’ oversight role.”

The groups further pressed the intelligence community to report the data and “provide a public explanation for the increase in the number of call detail records being collected.”

In a separate letter to Judiciary Committee leaders Bob Goodlatte Robert (Bob) William GoodlatteNo documents? Hoping for legalization? Be wary of Joe Biden Press: Trump's final presidential pardon: himself USCIS chief Cuccinelli blames Paul Ryan for immigration inaction MORE (R-Va.) and Jerrold Nadler (D-N.Y.), the groups urged Congress to “use all the tools at your disposal to ensure that ODNI and NSA report this number as required by the USA Freedom Act.”

The call detail records are collected under Section 215 of the Patriot Act, which is due to expire next year. Lawmakers are poised to soon begin discussing whether to renew or amend it.

The report issued in early May disclosed that the NSA collected 534 million call records in 2017, over three times the number collected in 2016. The report suggested that the call detail metric is likely overstated given that the government counts each record separately, even when it receives the same record multiple times from different providers.

The report acknowledged the requirement to provide the count of unique identifiers under the USA Freedom Act, but said “the government does not have the technical ability to isolate the number of unique identifiers within records received from the providers.”

At the time, a DNI spokesman said that a range of factors could influence the number of call records collected and the agency anticipates the numbers will “fluctuate from year to year.”

“These factors include the number of Court-approved selection terms — like a phone number — that are used by the target; the way targets use those selection terms; the amount of historical data that providers retain; and the dynamics of the ever-changing telecommunications sector,” Tim Barrett, the spokesman, said.

A spokesman for the Director of National Intelligence said the office had received the letter and would respond.

This post has been updated.