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
There's no doubt that Uber is innovative and entrepreneurial, writes Joel Hills - but it's also ruthless and aggressive.
Having now heard Theresa May's Florence speech, I have a bit more understanding why Boris Johnson cut up rough last weekend.
As Theresa May unveils plans for a two-year transition period, and Nigel Farage says she's 'betrayed' Leave fans...what do voters think?