I have operated an ammonium fertiliser plant larger than this. there appears to be a lot of misinformation. Anhydrous ammonia is not explosive under any circumstances. Ammonium Nitrate is also not explosive unless it is mixed with diesel or some other inflammable, and then detonated. For example, my local fertiliser supplier has 6 tonnes of bagged ammonium sulphate on the floor in the warehouse, open to the public and within a densely populated area, thus demonstrating this.
However buying it requires a whole raft of forms etc, even foe me, a qualified industrial chemist with 50 years experience in the industry, to prevent access by terrorists.
What is highly explosive however, is liquid Hydrogen, one of the two key raw materials. This is usually formed from natural gas or by electrolysis of water. It is then stored on site in pressure vessels as liquid. I can see these on the Google Map of the site. As such it is equivalent to rocket fuel and in fact was used as such in the space shuttle program.
If the explosion had involved anhydrous ammonia and/or ammonium nitrate as the cause, nobody within at least a 1 mile radius would have survived, due to suffocation or choking from toxic fumes.
Therefore I think this was almost certainly a terrorist attack in an attempt to gain access to significant ammonium sulphate solid. If just 20-30 kg (1/2 a bag) had been available and been used in the pressure cooker bombs in Boston, combined with diesel and detonators/mobile phones the death toll would probably have been hundreds. They are bound to have been disappointed with the result. The fact that nobody has claimed credit suggests the the mission is still not completed!