WASHINGTON - The salmonella strain linked to a nationwide outbreak has been found in irrigation water and a serrano pepper at a Mexican farm, federal health officials said Wednesday.

Dr. David Acheson, the Food and Drug Administration's food safety chief, called the finding a key breakthrough in the case, as did another health official.

"We have a smoking gun, it appears," said Dr. Lonnie King who directs the center for foodborne illnesses at the Centers for Disease Control and Prevention.

Acheson said the farm is in Nuevo Leon, Mexico. Previously, the FDA had traced a contaminated jalapeno pepper to a farm in another part of Mexico.

Acheson and other officials were grilled 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 has sickened more than 1,300 people since April.

Tomatoes had been the prime suspect in the nationwide outbreak for weeks. But last week, the FDA said only jalapeno peppers grown in Mexico were implicated in the nationwide salmonella outbreak. The FDA said then it had found the same strain of salmonella responsible for the outbreak on a single Mexican-grown jalapeno in a south Texas produce warehouse.

If it turns out the tainted irrigation water was also used on tomatoes, it could provide some of the evidence that federal authorities are looking for to back their original focus on the fruit.