As for the lack of a terrorist group claiming credit, keep the self-radicalized concept in mind. The perpetrators of the Paris attacks weren't working under direct control or orders from DAESH, they were acting on their own operational control.
If... IF... there ends up being a terror element to this, it might be some self-radicalized person or cell behind it, inspired by DAESH, BUT working independently of DAESH or whomever. DAESH, or whomever, won't "claim credit" for this until after it's actually proven to be a bomb.
If they were to claim credit for this, and it ends up being a technical issue that caused it, it would discredit the organization who claimed credit.