The salmonella strain linked to a U.S. outbreak has been found in irrigation water and in a sample from a batch of serrano peppers at a Mexican farm, health officials said Wednesday.

Dr. David Acheson, the United States 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 Tamaulipas, Mexico. Both farms shipped through a packing facility in Nuevo Leon, raising the possibility that contamination could have occurred there.

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

Industry representatives complained that they have lost more than $300 million US and had to dump tons of good tomatoes they could not sell because of government warnings.

The probe was slowed even more because FDA investigators were unfamiliar with the workings of the tomato industry and were reluctant to share information, they said.

More than 1,300 people sickened

But federal officials insisted that tomatoes still cannot be ruled out and that it is quite possible the outbreak was caused by several 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 currently implicated.

The FDA said it had found the same strain of salmonella responsible for the outbreak on a single Mexican-grown jalapeno in a south Texas produce warehouse. The agency explained that any contaminated tomatoes would be out of the food supply chain by now.

For now, the focus of the investigation is on the two farms in Mexico, which Acheson said are quite far from each other.

The Tamaulipas farm also grew tomatoes and peppers, Acheson said. But the tainted pepper traced to that farm was found at a warehouse facility in McAllen, Texas, raising the possibility it could have been contaminated along the way.

Acheson said samples have been taken from the Tamaulipas farm, and lab results are pending.

The Nuevo Leon farm did not grow tomatoes.