I don't believe it's accurate to speak of "several international aviation agencies." There's ICAO. And it's the only game in town.
IATA is a trade association; IFALPA an organized labor entity; ACI an airports industry group, etc.
Annex 13, under which investigations are conducted, itself isn't exactly "law." The Chicago Convention of 1944 is an instrument within international law, but the extent to which any Annex is "law" gets into pedantic territory (and even more so regarding SARPs....).