The powers granted to the National Security Agency to spy on millions of Americans and people abroad were vested by a little-known executive order that—until now—has received scant scrutiny or oversight, newly uncovered government documents revealed on Monday.

Executive Order 12333, passed in 1981 by President Ronald Reagan, is the “main game in town for NSA surveillance,” according to Alex Abdo, staff attorney for the American Civil Liberties Union, which obtained internal documents on the order through a Freedom of Information request.

One of the documents, an internal surveillance manual published by the NSA, describes EO 12333 as the “primary source” of their intelligence-gathering authority. And a “Legal Fact Sheet,” distributed by the NSA two weeks after Edward Snowden disclosed their widespread surveillance, says that the agency conducts the majority of their intelligence gathering through signal interruption (or SIGNIT) “pursuant to the authority by EO 1233.”

Unlike Section 215 of the Patriot Act or the FISA Amendments Act—which thus far have been the focus of public debate—the executive branch is alone in implementing EO 12333, meaning that there is essentially no oversight from Congress nor the court system.