The National Security Agency has been spying on bitcoin users around the world beginning as early as March 2013, according to a story published by The Intercept.

Classified documents leaked by whistleblower Edward Snowden show the NSA used a secret data source that “leveraged the NSA’s ability to harvest and analyze raw, global internet traffic while also exploiting an unnamed software program that purported to offer anonymity to users.”

The NSA reportedly tracked bitcoin users by collecting sensitive information from their computers, including passwords, internet activity, and unique identifiers assigned to devices known as MAC addresses. Documents also suggested that the agency used XKeyScore, the NSA’s formerly secret global internet data analysis program, to monitor targets tied to bitcoin.

According to the documents, the tracking of bitcoin users as of 2013 was done through OAKSTAR, a program that consisted of a “collection of covert corporate partnerships enabling the agency to monitor communications, including by harvesting internet data as it traveled along fiber optic cables that undergird the internet.”

MONKEYROCKET, a sub-program of OAKSTAR, was used specifically to track bitcoin users, was operational on every continent except for Africa, and documents suggest the program was at one point the only way to surveil bitcoin users. According to The Intercept, MONKEYROCKET functioned as a privacy bait and switch, disguising itself as a tool to provide Bitcoin users with anonymity online while in reality sending their information to the NSA.

Documents show that although the NSA was also interested in surveilling the users of other cryptocurrencies, bitcoin was their first priority.