[Species8472]

I can't say weather it was a bomb or not, but very clearly, 15 seconds into the recording the firefighter on location states that a bomb just went off in the building.

It takes a high explosive to initiate a low explosive like ammonium nitrate.

[reaganaut]

Not really we used to blow up containers of the stuff on the farm or use it to demolish things, it just needs an igniter or good size spark.

[Bidimus1]

It does not require a High Explosive.

http://www.sinooan.com/do-you-know-much-of-disaster_91.html

http://www.hysafe.org/science/eAcademy/docs/3rdfesshs/presentations/ESSHS2008DorofeevSB.pd

Texas City 1947 is an example of this phenomena.

Prayers to the survivors and those that have lost loved ones.