Comment by efitz
> a clandestine explosive installation would have resulted in far greater damage and included shrapnel
No and not much. The amount of explosion you get is proportional to the amount of explosives used. Small amount of explosives == small explosion.
Shrapnel is specifically engineered into explosive military weapons - it is not an innate property of explosive reactions. If you want a lot of shrapnel you have to design the case to fragment (e.g. grenades) or pack the area around the explosive with the stuff you want to become shrapnel (as with many bombers packing nails and screws and bolts etc., around their bombs). A small explosion in a mostly plastic device will result in a small amount of small pieces of plastic being scattered, which might harm bystanders but is by no means guaranteed or even intended to do so.
It would seem if they're going to all this trouble in the first place to design a substitute case of materials that had good shrapnel effects.