How Were The Bombs Detonated?

I haven't been able to find anything, just speculation. If it were cell phones that detonated them, are there phone records of the two suspects calling out at the time of the blast? I see the pic of the white hat guy putting a bag down behind the 8 yr old kid, so if it really was a bomb in there, how did he set it off?
Could it have been a timer? If so, how long after the bombs were placed did they explode? Is there any photographic evidence of either of the suspects getting in their bags, possibly starting a timer?
To me, this is a very important part of the puzzle. Did the two suspects set the bombs down at the same time, rendezvous at a certain location and hit a switch or make a call? Or, did they head off in different directions and plan something like "when you hear mine explode, wait a minute and call (or press) yours"?
Which of the two suspects' bombs detonated first? Is there any evidence that a cell phone was used for the detonation? In the pictures of the bomb fragments, I see nothing that indicates a phone, just a couple of batteries and a radio shack electronics board (and of course the remnants of the pressure cooker). Why do they keep pushing the cell phone theory?

Any clarification would be greatly appreciated...