I wonder if an autopsy would show any type of poison.
Lots of poisons can only be found if someone is actively looking.
The castor oil plant seeds can be used to make ricin pretty easily. I just looked it up; it grows anywhere tropical/subtropical.
The symptoms look like typical food poisoning/travels’ stomach.
Just looked; ricin is also basically impossible to test for. They have to look for anti-bodies.
I wouldn’t be surprised if the DI has a serial killer on the loose.