...but the green key in the side of the device looks like the kind of key in that .pdf, doesn't it?
Necklace bombs like this have been used to coerce people into committing crimes before, notably in Colombia. (by the FARC). None of the bombs use there appears to have been as sophisticated as this infernal machine.
The FBI have a lot to go on, though. They have the remains of the bomb, they have the autopsy on the unfortunate Mr Wells, they have the many interviews they are doing, and they have the note.
If this is Wells's own doing, he's smarter that people seemed to think, but too smart for his own good. I don't think a guy who didn't have jhis own workshop would be able to make that collar, the crude metalwork notwithstanding.
It looks to me like the collar bit was some kind of automotive bracket, and the box part of the bomb was built-up. I wonder if there is any significance to the blue paint? The guy who built the box part had the following tools at a minimum:
d.o.l.
Criminal Number 18F
I think, as investigators, they are just considering it still possible. Until proven otherwise they have to. I don't think they could have done anything more than they did. Time was too short, the situation too strange and volatile and the device was too complex and arcane.