An audio message from the wife of an official (private WhatsApp group) claims the blast was due to 'utter negligence'.
Ammonium Nitrate was 'salvaged from a sinking ship' about 12 years ago and stored at 'Hangar 12, one of #Beirut harbour's biggest'.

A 'health and safety' check done 6 months ago revealed 'the extremely dangerous and volatile circumstances' the NaNo3 was stored in.
'This is extremely volatile material which is liable to ignite at any time, especially in circumstances of excessive heat or humidity'

She quotes her husband as having told her what the health and safety report said at the time.
Unfortunately, our entire harbour is on fire, so I don't think an independent verification can authenticate the ongoing NaNo3 salvage narrative.