Like undercover officers hang out in feed stores. Smells like LE knew something before hand. IIRC, there was an increase in security at the game as well.
Also, can anyone confirm the remote detonation device that went BOOM was the one on his body or could it be that someone else pushed the button when he couldn't get into the stadium and might have been having second thoughts?
I'm sure all these fertilizer places are aware of the possibility of the product they sell being used for explosives.