Let's say someone (and his or her buddies) is a creatively driven terrorist, who literally wants to fly under the radar. It's not a stretch to think someone could pack propane tanks with nails, screws, etc (innocently purchased from a local home improvement store).
And let's say these charming, lovely people with the nail filled propane tanks also packed some explosive material inside-- the list of explosives is only limited by imagination. And duct taped an ignition source to the outside-- also only limited by their imagination, ambition, and budget..
Then let's say our would-be terrorists put a number of the easily loadable propane tank bombs into their little bitty civilian fixed wing aircraft, and go for a rather low flying ride (VFR), so as not to have to file detailed flight plans, or communicate with towers, etc. Like "touch and goes". But then they deviate and fly low over a densely populated area or activity-- like an outdoor stadium filled with spectators, or a freeway at rush hour, or something. And kind of dropped the propane tanks out the door of the little bitty civilian aircraft, while activating the ignition source.
See the picture? You don't need a cannon to launch an improvised explosive. A terrorist is not necessarily going for precision accuracy.
Heck, a hot air balloon would work.
Well, and if we take that a step further, and imagine that the pilot and his/ her companion don't mind dying for their cause/ mission, imagine what a little bitty airplane filled with propane tank bombs could deliver as a kamikaze mission.
A barrel bomb is an improvised unguided bomb, sometimes described as a flying IED (improvised explosive device). They are typically made from a large barrel-shaped metal container that has been filled with high explosives, with possibly shrapnel, oil or chemicals, and then dropped from a helicopter or airplane.[1] Due to the large amount of explosives (up 1,000 kilograms), their poor accuracy and indiscriminate use in populated civilian areas (including refugee camps), the resulting detonations have been devastating.[2][3][4] Critics have characterised them as weapons of terror and illegal under international conventions.[5]
https://en.wikipedia.org/wiki/Barrel_bomb#United_States
And it's not particularly believable to me that there was NO surveillance cameras at the gas stations where the propane tanks were stolen. I think they have surveillance of whomever was brazen enough to steal the tanks-- even if they were only stolen one at at time, over days or weeks. JMO.