The outbreak came about six weeks after hepatitis A outbreaks in Tennessee, Georgia and North Carolina.
Investigators think the four outbreaks could be related because of similarities in the viral strains, but finding a theory of contamination that would explain the time gap has proven elusive, said Jack Guzewich, director of emergency coordination and response in the U.S. Food and Drug Administration's Center for Food Safety and Applied Nutrition.
The chances of identifying the human sources of contamination in the outbreaks is somewhere between "zero and nil," Guzewich said. Even the chances of pinning the contamination problem to a particular farm aren't great, he said.
"More than likely, we'll be able to identify categories of practices that were an issue -- water in the field, water in the packing shed, ice or workers," Guzewich said. "If we can get to a category ... we'd be satisfied."
There are eight investigators overall -- three from the FDA, one from the Centers for Disease Control and Prevention and four from the Mexican government.
The number of people sickened in the Beaver County outbreak remained at 615 yesterday.
Also yesterday, a Beaver County couple filed suit in U.S. District Court in Pittsburgh against three California green onion suppliers and a fourth in Kentucky. All four companies were identified as onion suppliers to the Beaver Valley Mall Chi-Chi's.
Richard Miller of Beaver, who survived a liver transplant after the outbreak but remains hospitalized, and his wife, Linda Miller, named four defendants: Newstar Fresh Foods of Salinas, Calif.; Apio Fresh of Guadalupe, Calif.; Boskovich Farms of Oxnard, Calif.; and Castellini Co. of Wilder, Ky.
The complaint is the first to accuse the onion suppliers, and the first filed in a federal court.
Three state court suits -- two in Beaver County and one in Allegheny County -- named Chi-Chi's as the defendant, but those were voluntarily withdrawn because of a bankruptcy court stay issued in Delaware. Chi-Chi's is in bankruptcy.
"Those can and will be refiled when we're permitted to do so," said attorney Jerry Meyers of the Downtown law firm of Meyers Rosen Louik & Perry, which filed suit in conjunction with Seattle firm Marler Clark.