I’m really surprised fed didn’t check or remove the bins before the event, like we often do here and have done for 30 years when alert levels are high. Ok there is litter thrown on the ground but then get a council worker to sweep it up. and they also become a extra pair of eyes to watch for unusual behaviour.
Unfortunately there are a lot of people who want to set off bombs at public events for all manner of reasons, and its pointless trying to prejudge or suspect any one group as it diverts security resources from other suspects, whereas frontline monitoring does work (for instance there have been numerous bomb attempts still in NI but surveillance and detective work have meant devices have usually got defused before anyone got hurt).
Even for SIA training (“bouncer training”) for a English nightclub part of the module covers (minor) IED and CBRN threats.
