There were loads of people - members of the general public, security researchers, government watchdog types, privacy advocates, crazy conspiracy nuts, etc. - who very strongly suspected, for good reason, exactly what turned out to be going on.
ECHELON started in the 1960s. Rumors about it were everywhere by the early 1990s. It became so famous it was featured in pop-culture movies, TV shows, and video games.
There was at least one good book (note 2005 publication) that showed how it was possible to piece together some pretty good guesses about what was happening from unclassified information:
In short, that book argues the NSA was expanding its eavesdropping capabilities so enormously, so quickly, that the only reasonable target for it was "everything." There simply weren't enough top-secret, diplomatic, or encrypted messages to justify the infrastructure devoted to the task; the NSA had to be developing the ability to listen to absolutely anything it wanted to.
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