it is hard to say i have any evidence *sigh* cause i was not there but according to the songs there are things we can infer or deduce to narrow down the possibilities.
in truth i am not a detective, and have no experience in deducing the truth from little to no evidence. however the two i would take into the questioning room first would be Furuto, and Bee.
little to nothing is said yet about these two but in Bee's character description is that she is only there to help her friends. (how her mind thinks she is helping them is what i am worried about). and Furuto is not a leader of the other group and there is a lot a "side character" sees and doesn't really tell anyone about.
as for my #1 suspect always Kanesaki Hyudo, but i wouldn't trust Nori either.
my gut tells me that i am wrong though. i am afraid there is a feeling, that one of the idol girls took part in this bombing where that leads my mind to at the moment idk. but i look forward to a great story from BSS anyway it plays out.