On November 21, 2013, a cargo ship arrived in the port of Beirut, carrying 2750 tons of ammonium nitrate. The authorities all plead ignorance of the affairs of the port at that time. But let's recall what was happening in Lebanon in the preceding months. (A brief thread)
July 2013: The CIA allegedly warned the Lebanese authorities about an Al-Qa'ida plot to smuggle several tons of explosives into Beirut. https://qifanabki.com/2013/07/17/when-langley-met-dahiyeh-a-trans-oceanic-love-story/
August 2013: A major blast in the southern suburbs of Beirut, and a defiant response by Hizbullah. https://qifanabki.com/2013/08/16/pathos-and-promises-nasrallah-on-the-dahiyah-blast/
Also in August 2013: a pair of bombs targeting Sunni mosques in Tripoli. https://qifanabki.com/2013/08/24/motives-mysteries-tripoli-edition/
Also in August 2013: Obama announces violation of red line in Syria, and that US would soon be launching airstrikes: https://qifanabki.com/2013/08/28/operation-stage-whisper/
November 19, 2013: Suicide bombing in front of the Iranian embassy in Beirut. https://en.wikipedia.org/wiki/Iranian_embassy_bombing_in_Beirut
All this is to say that the months leading up to the arrival of the Rhosus in November 2013 witnessed great turbulence in Lebanon and a heightened degree of vigilance among all the security agencies. The notion that the Port of Beirut wasn't under close scrutiny is untenable.
Was the ammonium nitrate smuggled to Beirut? Or did it end up in the port purely by accident? It doesn't matter. The question is why it was left untouched for years after the fact, even though the security apparatus surely knew about it. This is the question.