Birds, pigs, water, air — How did the bacteria find the romaine?

Investigators continue their tedious search for the source of the E. coli that apparently contaminated romaine lettuce, causing this spring’s deadly outbreak. But it’s becoming increasingly unlikely they will find a smoking gun.

The human toll so far, including one person in California who died, is 132 confirmed cases, 72 hospitalizations, and 20 people with hemolytic uremic syndrome, a type of kidney failure, according to the Centers for Disease and Prevention.

Epidemiologists from the CDC are working closely with investigators from states and the U.S. Food and Drug Administration. They are all trying to find common denominators among victims and entities along the farm-to-fork supply chain that will help pinpoint the source of the dangerous E. coli O157: H7 bacteria.