I find it a little confusing that their parents claimed that the brothers were being monitored by the FBI so couldn't have planned the attack, which I'd initially disbelieved, and then is was revealed that they were interviewed by the Feds in 2011 so the parents' claim must have credence.
I could understand that people that may have once been considered a risk, somehow got under the radar and carried out a horrendous act or whatever, but these guys seemed to be armed to the teeth with explosives and other lethal weapons. How could that occur?