Thallium is element 81. A heavy metal banned in the United States since 1972. Colorless, odorless, and tasteless in its soluble form. It mimics potassium so convincingly that your cells invite it in. It starts with nausea, fatigue, stomach pain — things that look like a hundred other conditions. Weeks later, the hair falls out in fistfuls. By then, the nervous system has already taken serious damage.

The antidote is called Prussian blue. Doctors almost never think to use it, because they almost never think to test for thallium. That gap between exposure and diagnosis is exactly what makes it a weapon.

In the fall of 2017, a woman named Brigida started feeling sick in her home in Dulzura, California. She went to doctor after doctor. She was told it was fibroids. Cancer. Lupus. An autoimmune disorder. One doctor told her she probably ju ...