The attacker in London was home grown. No one let him in.

Same with the guy who attacked Canadian Parliment in 2014. He killed a Canadian soldier standing watch of the tomb of the unknown soldier, prior to going on a shooting spree inside parliament and being taken down by RCMP and parliamentary security.

[Link: en.m.wikipedia.org]

John Kaczynski, Timothy McVeigh, home grown.

These types worry me more than imported threats.