The U.S. National Security Agency (NSA) collected more than 500 million U.S. call records in 2017, Reuters news agency reported. This was stated in the agency’s official annual report released today.

A total of 534 million phone call records of Americans was gathered in 2017. In comparison, the NSA gathered 151 million records in 2016.

According to Reuters, the increase occurred during the second full year of a newly established surveillance system. The system was established in 2015, following the passing of a new law seeking to limit the agency’s ability to collect records in bulk. The reason for this spike was, therefore, not immediately clear.

In a statement, Timothy Barrett, a spokesman at the Office of the Director of National Intelligence, explained the increase, stressing the NSA has found that a number of factors may influence the amount of records collected.

“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. We expect this number to fluctuate from year to year.”

Mr. Barrett also said that the agency “has not altered the manner in which it uses its authority to obtain call detail records.”

The NSA used to gather nearly 5 billion phone records a day. This was revealed by the Washington Post in 2013, following the Snowden Leaks. The gathered phone records would feed the NSA’s database, so the agency would be able to track movement of targeted individuals, mapping out their relationships in the process.

Featured image credit: Charles Platiau AP Images

Edward Snowden is a former contractor for the United States government who copied and leaked classified information from the NSA. The information was made available to the public with the help of journalists Glenn Greenwald, Laura Poitras, and Ewen MacAskill. Explaining the NSA’s power and ability to The Guardian, Snowden said the following.

“The NSA has built an infrastructure that allows it to intercept almost everything. With this capability, the vast majority of human communications are automatically ingested without targeting. If I wanted to see your emails or your wife’s phone, all I have to do is use intercepts. I can get your emails, passwords, phone records, credit cards.”

In that sense, the congressional limits on surveillance seem to have worked. The USA Freedom Act, the 2015 bill that has limited the NSA’s ability, was praised by Snowden himself, although he had also urged further congressional limits.

The legislation was signed into law by President Obama and requires intelligence agencies to obtain a warrant from a counter-terror court before accessing data. As Time noted, the act also renews roving wiretaps and tracking of so-called “lone wolf” suspects.

At the moment, according to the agency, the metadata records include the phone numbers and time of a call, but not its content.