The National Security Agency’s ability to spy on vast quantities of Internet traffic passing through the United States has relied on its extraordinary, decades-long partnership with a single company: the telecom giant AT&T.

While it has been long known that American telecommunications companies worked closely with the spy agency, newly disclosed NSA documents show that the relationship with AT&T has been considered unique and especially productive. One document described it as “highly collaborative,” while another lauded the company’s “extreme willingness to help.

AT&T says it has stopped using a controversial mobile technology that could be misused by advertising networks to track online users regardless of their wishes. Until last week, the company had been inserting a unique identifier in web traffic sent by phones and other devices on its wireless network.

It was doing this as part of a test program, which has now been stopped. Privacy advocates hate these unique identifiers, because there’s no way to turn them off. That means that they can be used by advertising networks to circumvent privacy tools such as do-not-track lists or private browsing settings.

US law enforcement officers working on anti-drugs operations have had access to a vast database of call records dating back to 1987, supplied by the phone company AT&T.

The project, known as Hemisphere, gives federal and local officers working on drug cases access to a database of phone metadata populated by more than four billion new call records each day. Unlike the controversial call record accesses obtained by the NSA, the data is stored by AT&T, not the government, but officials can access individual's phone records within an hour of an administrative subpoena.