jeff lapoint was keeping his mind open, letting the clues lead him to a theory about what >>> like any good detective, dr. lapoint was keeping his mind open. letting the clues lead him to a theory about what might be killing brigida. >> i've got to figure out what's wrong and get the treatment started. >> the symptoms could've been caused by range of toxic chemicals like arsenic, cadmium. but to the dr. lapoint one particular symptom stood out. sudden hair loss. >> that's the telltale sign. any medical toxicologist, if you have a young woman who has lost her hair and now cannot walk because her feet hurt so bad, thallium will be on the list. it's straight out of the books. >> thallium is a heavy metal. atomic number 81 on the periodic table of elements. >> it was discovered late 1800s and quickly began being used medically early 1900s. it's a depilatory agent. if you had lice and you wanted them to lose their hair, you would give them thallium. >> for decades, thallium had another use. as an ingredient in rat poisoning. >> it was good for that. that was in the 1970s. it is good at kill