i don't see how this proves the bombing was an inside job. Just because they knew before hand doesn't mean they were complicit, or that they had the power to prevent it.
all they had to do was check garbage cans at the finish line
yep.
Or.. they could have already checked the can before and found nothing, and the perpetrator waited for his moment to strike. We can't always assume complicity here. But, they should have been running continuous sweeps throughout the event.