The National Security Agency collected more than 530 million U.S. call records in 2017, representing a dramatic increase over the previous year.

According to an annual transparency report released Friday, the spy agency collected 534 million call records in 2017, more than three times the 151 million collected in 2016. The new statistics were first reported by Reuters.

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The report, released Friday by the Office of the Director of National Intelligence, is mandated by the USA Freedom Act passed by Congress in 2015 that aimed to restrict and boost oversight of the spy agency’s surveillance program.

The NSA’s surveillance powers have weathered scrutiny since the 2013 Edward Snowden revelations that exposed the agency’s now-defunct bulk collection program. Under the old program, the NSA is estimated to have gathered billions of phone records.

The call detail records, obtained from U.S. telecommunications providers, include the numbers and the time and duration of the phone calls. They do not include the actual content of the call itself. They are collected under the Foreign Intelligence Surveillance Act (FISA).

A spokesman for the Director of National Intelligence office told The Hill that the NSA has discovered that a range of factors can influence the number of call records collected and that the agency expects them to “fluctuate from year to year.”

“The government has not altered the manner in which it uses its authority to obtain Call Detail Records pursuant to FISA. Rather, the NSA has found that a number of factors may influence the number of Call Detail Records that NSA receives,” said Tim Barrett, a spokesman at the Office of the Director of National Intelligence.

“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,” Barrett said.

The report suggests that the call record metric is likely overstated because the government counts each record separately even when it receives the same record several times, such as from different providers.

“Additionally, this metric includes duplicates of unique identifiers — i.e., because the government lacks the technical ability to isolate unique identifiers, the statistic counts the number of records even if unique identifiers are repeated,” the report states.