Raffaello Pantucci, from RUSI, prompted by two high-profile cases, involving lone extreme right wingers in the past week, reminds us to be blunt they talk or announce their intentions beforehand:
Preventing terror attacks by lone individuals poses a serious challenge. But there are sometimes behaviours and actions that might give them away....Our research suggests that, more often than not, lone actors imagine that they belong to a wider movement – sometimes attending group activities such as rallies and conducting online research....Individuals had leaked information about their plans in about half of all cases.

The article refers to a RUSI project for the details:https://rusi.org/publication/occasio...m-final-report