Radicalization done by FBI informant/employee.
They gave the kids the idea, the materials and coached 'em through it. Something went wrong in the plan, or the plan changed last second to live explosives.
It's not uncommon, unusual, and has happened before.
It's almost ALWAYS the FBI, I don't need to post ref links here, you guys all know this stuff.
So who is responsible? My opinion, the FBI. But we don't know yet, do we?