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
Nabi Tajima, who was the last known person to have been born in the 19th century, died in Japan after being in hospital since January.
Temperatures are predicted to reach a high of 23C.
Hunt accused companies of "turning a blind eye" to a generation of children "exposed to harmful emotional side effects" of social media.