Ever since legendary British investigative journalist Duncan Campbell told the world in a 1988 magazine article about ECHELON — a massive, automated surveillance dragnet that indiscriminately intercepted phone and Internet data from communications satellites — Western intelligence officials have refused to acknowledge that it existed.

Despite sporadic continuing press reports, people who complained about the program — which, as Campbell disclosed, automatically searched text-based communications using a dictionary of keywords to flag suspicious content — were routinely dismissed as conspiracy theorists.

The only real conspiracy, it turns out, was a conspiracy of silence among the governments that benefited from the program.

As Campbell writes today, in a first-person article in The Intercept, the archive of top-secret documents provided to journalists by NSA whistleblower Edward Snowden contains a stunning 2005 document that not only confirms ECHELON’s existence as “a system targeting communications satellites”– it shows how the program was kept an official secret for so long.

It describes how in 2000, the European Parliament responded to increasingly authoritative reports that ECHELON was being used to indiscriminately survey non-military targets — including governments, organizations and businesses in virtually every corner of the world — by appointing a committee to investigate the program.

Members of the committee vowed to get the truth from the NSA. What happened, according to an article in the NSA’s own in-house “Foreign Affairs Digest” was this:

Corporate NSA (FAD, SID, OGC, PAO and Policy), ensured that our interests, and our SIGINT partners’ interests, were protected throughout the ordeal; and ironically, the final report of the EU Commission [link] reflected not only that NSA played by the rules, with congressional oversight, but that those characteristics were lacking when the Commission applied its investigatory criteria to other European nations.

The initials there stand for NSA’s Foreign Affairs Directorate, Signals Intelligence, Office of the General Counsel, and Public Affairs Office.

And then, in what is possibly one of the most memorable lines to come out of the Snowden archive, the author of the article, a “foreign affairs directorate special adviser,” concluded with this observation:

In the final analysis, the “pig rule” applied when dealing with this tacky matter: “Don’t wrestle in the mud with the pigs. They like it, and you both get dirty.”

ECHELON was the precursor to today’s worldwide dragnet, which thanks to Snowden, we now know is carried out by tapping the massive fiber-optic cables that encircle the globe, in addition to the satellites that orbit it. It was the collect-it-all of its time.

As it happens, not every one of the ECHELON conspiracy theories turned out to be substantiated. On “Jam Echelon Day” in October 1999, people around the world sent a huge volume of communications over the Internet and over the phone using as many of the presumed Echelon keywords as possible.

But the Snowden archive contains no evidence that the NSA routinely scanned voice communications for keywords back then. That’s something they’ve only gotten good at recently.

The story of Campbell’s four decade long career exposing mass surveillance — including his introduction to the world of the organization known as GCHQ — is a great read. Make time for it.