In any case, nearly 30 people have lost at least one leg in this attack.
Fox is showing a suspicious package on the ground at the point of explosion. If the second bomb was on the ground, the leg injuries would be expected.
I would say that the bomber wanted to maim and not kill, but I think it is more likely that the motivation for placement on the ground was that it was the easiest place to leave the pack wihout attracting attention.