 Complete the following phrase:  According to Gilles de Kerchove in 2017, the UK had the highest number of Islamist radicals in the EU numbering between 20 and 25,000. Of those, 3000 were considered a direct threat by MI5 and 500 were under constant surveillance. Among those known to security services but not considered an immediate threat were the terrorists of three ISIS-linked attacks in 2017 