Will be interesting to hear about how the brothers are connected to the people in the supermarket incident and whether this was a coordinated attack.
I kind of feel like they weren't actually connected, but the supermarket guys were just opportunists seeing a chance to piggyback another attack. This is based on the brothers clearly being so inclined towards martyrdom, yet the supermarket attackers were seemingly bargaining for their safety when they threatened to kill hostages if the warehouse was targeted.
Speculation. Time will tell.