[weston]

My thought is that these two brothers were resposible for second bomb.
Others set off 1st one. The younger brother carried the actual bomb. Older brother set it off.

[Mozilla]

What is so odd is that they didn’t flee after planting bombs. They stayed around to see what they did. As people ran away they quietly walked away after the explosions at the marathon.

[no-to-illegals]

these two brothers were responsible for second bomb.

not to disagree though am thinking first bomb.