The salmonella strain linked to a nationwide outbreak has been found in irrigation water and a serrano pepper at a Mexican farm, and Dr. David Acheson, the Food and Drug Administration's food safety chief, is calling the finding a key breakthrough in the case.

Acheson said the farm is in Nuevo Leon, Mexico.

The FDA earlier traced a contaminated jalapeno to a farm in another part of Mexico.

Acheson and other officials were grilled last week at a congressional hearing about why the investigation originally focused on tomatoes.

The officials insisted that tomatoes still cannot be ruled out and that it is quite possible that the outbreak was caused by several different kinds of contaminated produce.

The outbreak sickened more than 1,300 around the country and more than 500 in Texas.