Police discovered fragments of a "silent conversation" writing in a document on Dart's laptop and then deleting the text.
Counter-terrorism teams also believe that the pair used the same tactic walking down the street with a mobile phone.
More top news
"Our ability to pre-empt or respond to threats will be eroded if we don't keep up with our adversaries."
"You are one of the silence breakers and we all owe you a debt of gratitude."
Maximum temperature 11 C (52 F).