Reply #6: Not surprising, only in detail. Witnesses reported the smell of sulfur, and ...

[eppur_se_muova]

bits of lead and silver ore thrown out of the ground. Sulfur and/or selenium (toxic) are often associated with heavy metals such as these (PbS is the major ore of lead), and arsenic is a frequent contaminant. The fact that people got sick almost immediately makes chemical poisoning (especially by inhalation) much more credible than either infection or radiation exposure, either of which would take longer to show effects. So poisoning by heavy metal ores, or compounds formed from them, such as H2S or maybe H2Se, should have been a first guess.

Silver actually forms a lovely ore containing silver, arsenic, and sulfur -- proustite, Ag3AsS4. http://en.wikipedia.org/wiki/Proustite