I have used Ammonium Nitrate for many years and as far as I know it will only explode when it has been contaminated with fuel oil hence the name ANFO.I would assume that it was not mixed and then confiscated and stored but it may have been and would have been extremely dangerous as reports were that it had been there for 6 years.Usually we only mixed what we needed and made sure there was no mixed nitro left at the end of the day. I can't believe that is was stored even unmixed in a built up area let alone if it had been mixed but in some of these countries the storage facilities are fairly basic and it could have come in contact with fuel so it may have been deliberate or just negligent.
