So I am surprised there seemed (in many of the other pictures) to be so many men (including a lot of Arabic looking folks) with the same or very similar backpack. black with the stripes etc. do you think maybe there was a group and these guys were the ones who did the drop?
decoys?
Probably terrorists just watched the video of last year's Marathon and picked equipment that would blend with the crowd.
As someone explained yesterday, backpacks are common at the Marathon because people need to carry clothes and supplies for runners. Since backpacks are heavy, it is not unusual for people to put them down wherever they can. Perhaps an unattended backpack would become suspicious eventually anyway; so the bombers had to be quick with deployment and detonation - they just barely got away themselves; and as I understand the Saudi guy was still too close.