Concur with most of that analysis anon. It was definitely a shaped charge, but I'm not sure about the delivery. It could have possibly been an improvised shaped charge, and the instruction manual (Vietnam-era SF Handbook) has been in surplus stores and online for decades. My list of possible suspects include;
(Civilian)
Lone-wolf former SF
Extremely well trained and led Militia
Lucky gifted amateurs
(Gov't)
CIA direct-action paramilitary contractors
FBI
ATF
SOCOM (Joint Delta/RRC/ISA mission)
The odds of it being EhrmaGerd Patriots with some old TNT is zero.