I;m not so sure it’s that important at this point...not with all the evidence they have....the guys going to at minimum get life....might even try to plead insanity and under the control of his older brother.
The larger issue is getting what information they can....because since he is now in custody he’s no longer a threat to public safety.
it isn't about him, it's about his implements (does he have explosives stored in a storage unit somewhere, etc.) and knowledge of any other activities he was involved in that may be continued on by other parties.
It is not the bomber who is now a threat to public safety, just like my example in post 43.