After TS (top secret), two other letters repeatedly stand out throughout the leaked Pentagon files: SI. The letters stand for special intelligence, and signify that the information has been derived from signals intelligence work. This encompasses eavesdropping, surveillance and even backdoor access to information systems – all part of a panopticon of global, US-led intelligence collection.
Signals intelligence told the 1.25 million Americans with top secret clearance – and sometimes their British allies – that Russia had made marginal battlefield gains in Ukraine, that the Wagner group may be allowed to restart recruiting prisoners, and even that Russian hackers had gained control of a Canadian gas pipeline they hope to explode.
