Experts say young men and boys ‘with strong sense of grievance’ are ‘idolising’ shooters involved in US massacres. They are also falling through the gaps of UK terrorism laws

On the morning of 13 September, 18-year-old Nicholas Prosper was arrested while walking on a residential road in Luton. Minutes before, he had murdered his mother, younger brother and sister, shooting them dead in their family home.

Neighbours called police after hearing gunfire coming from the flat in Leabank tower, on Luton’s Marsh Farm estate, and officers found Prosper shortly afterwards on Bramingham Road. Later that day, searches of the area uncovered a loaded shotgun and more than 30 cartridges hidden in a nearby bush.