The U.S. National Security Agency collected more than 500 million phone call records of Americans last year, more than triple gathered in 2016, a U.S. intelligence agency report released on Friday said.

The sharp increase to 534 million call records from 151 million occurred during the second full year of a new surveillance system established at the spy agency after U.S. lawmakers passed a law in 2015 that sought to limit its ability to collect such records in bulk. The reason for the spike was not immediately clear.

The tally remained far less than an estimated billions of records collected per day under the NSA’s old bulk surveillance system, which was exposed by former U.S. intelligence contractor Edward Snowden in 2013.

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