Comment: But firefighters would have been on guard.

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In reply to comment: You're right (see in situ)

But firefighters would have been on guard.

They have tons of fertilizer at the Big Acre store, even at Walmart or Home Depot.

http://en.wikipedia.org/wiki/Ammonium_nitrate
Ammonium nitrate decomposes into the gases nitrous oxide and water vapor when heated (non-explosive reaction); however, ammonium nitrate can be induced to decompose explosively by detonation.

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