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
Giraffes could disappear from the planet if current rates of extinction continue, it has been warned.
Foreign Secretary Boris Johnson has been filmed accusing British ally Saudi Arabia of "puppeteering" in the Middle East.
Psychotic illnesses such as schizophrenia may be linked to immune system disorders, research has found - opening up new treatments.