Just to clarify, the bombs were 12 seconds apart. Not minutes, but not simultaneous either. They were timed when the peak number of runners were expected to finish, shortly after 4 hours from the start. On a packed finish straight, syncronised explosions would get the most casualties. A delay, even a short one, would see people move toward the original blast to help. The unsyncronised detonation would indicate the timers were not sophisticated. They were not detonated by mobile phone.