There was obviously a gap in security and Japanese special force experts are saying as such. Not letting suspicious people wander that close to the scene without questioning is pretty much an ironclad rule. Just like so called security details stand around yelling for ambulances and medical professionals several minutes afterwards while trying to lift the body themselves.
He didn’t appear to be a “suspicious” individual given others who were also standing around. Rather he looked like just another bystander among those behind Abe.
You just never know as they all walk among us every day....and with our border invasion now you can bet we won’t be able to tell which ones are going to act ciminally. In their cultures it’s often “normal” to take, rob, steal and rape.