With high-stress, high-stakes decisions, doctors around the world are frantically trying to figure out how COVID-19 is killing their patients so they can attempt new ways to fight back. One growing theory: In the sickest of the sick, little blood clots clog the lungs.

Poor couldn’t prove it. The tests required would further endanger his staff, who were already at risk of getting the virus. But the lung specialist saw clues that were “screaming blood clots.” So Poor pulled out a drug best known for treating strokes, and held his breath.

“I said, ‘What do we actually have to lose?’” Poor told The Associated Press. “That’s when I decided to give not just a blood thinner but a blood clot buster.”