Some of the unofficial reports speak of ball bearings or BBs in the bombs and of many severe lower extremity injuries, legs blown off and such. If this is all true, you have:

two devices going off almost simultaneously,

timed to go off when there were a maximum number of people around,

at a very high profile event,

constructed perhaps to broadcast metal fragments,

at a level where a lot of severe injuries are caused.

From this, my civilian, untrained self concludes, whoever did this was a pretty proficient terrorist, who either is a natural born genius who read a lot or was pretty well trained by people who knew what they were about.