Hmm, I've been following this one closely and it does seem to me like BDFLY is the most likely, especially considering fatalities with it have already been documented.
To those who say "some psychedelics" are totally non-toxic, this just isn't true. Moreover, psychedelics seem to be generally more toxic toward humans than rodents. Toxicity is ultimately a function potency and dose. Incidents like these, while very rare, unfortunately serve to document the mechanisms of toxicity: hyperthermia and cardiovascular issues including vasoconstriction appear to be the major ones.
I have a particular hypothesis to propose regarding relative toxicity of these things. Particularly, I recall that BDFLY showed higher binding affinity for 5HT2A than LSD in Dr. Nichols' studies, even though LSD is more subjectively potent in humans. I suspect that difference may be with respect to blood brain barrier permeability, a lack of which is intuitively consistent with a relatively long onset (note BDFLY ODs took 6+ hours to reach peak) and would most definitely accentuate the peripheral toxicity of the compound. Compounds like the DOXs, 2C-T-7 and what not also show this property. OTOH, poor BBB permeability and high peripheral toxicity may exist but not coincide with a long onset for chemicals with a very short overall duration of action, like many tryptamines (5-MeO-DMT comes to mind) but not like AMT or 5-MeO-AMT which are evidently rather toxic. This is just a simple idea of course, and onset could also be delayed by GI availability as well, but my bet is that relative duration of onset (for long acting chems) may be a clear marker for potential toxicity.
Take care.
EC, CC, etc...