You know, I once, eons ago, observed the symptoms of a certain heavy metal poisoning. It looked like a bad flu with paresthesias (pain in body, skin burning) and horrible tiredness. The fact that it was a heavy metal reminded me of "Agatha Christie's episode", and in fact, the symptoms that Agatha described might have saved a lot of people in the world, later, but if poisoned, one has to live into these symptoms emerging, they are not the first ones. If people die earlier, they'd die with the symptoms of some mysterious infectious disease, and absolute waste of energy (likely, cardiac problems; forgot, cardiac muscle needs to be tested for their presence).
That situation that I witnessed was so unusual, that I called to the Poison Control Centers in several countries, and heard that it was an "Oriental" way of poisoning. Also, i found out that one can gulp a portion of the poison and die immediately, or survive; alternatively, this stuff may be slowly added it to food/drinks and the end is worse.
It can even take years, as historic evidence tells us.
We shall know more; but spectroscopy of whatever tissues could be salvaged is interesting.
(I even wonder if these lattes...some food where the poison can be continuously added in tiny amounts...did any worker later leave that gas station for no reason?).
I also have a question: if the sisters said their food was tampered with, did they complain of metallic taste?
Here just in case the list of heavy metals
Learn about Heavy Metal Poisoning, including symptoms, causes, and treatments. If you or a loved one is affected by this condition, visit NORD to find
rarediseases.org