WASHINGTON (CNN) -- The salmonella strain linked to the recent outbreak has been found in irrigation water and a serrano pepper at a Mexican farm, a Food and Drug Administration official told lawmakers Wednesday.

The discovery is a "key breakthrough" in the investigation, Dr. David Acheson, the FDA's director of food safety, said at a congressional hearing.

FDA investigators had been investigating a specific farm in Mexico, Acheson said, to look for signs of the Salmonella Saintpaul outbreak.

"Two hours ago we learned that we had gotten a positive sample in both the water used for irrigation and a sample of serrano peppers from the same farm that match the outbreak strain," Acheson said.

The House hearing Wednesday had been called to look into the recent outbreak.

Last week, the FDA had said only Mexican-grown raw jalapeños and raw serrano peppers had been linked to the salmonella outbreak.

Mexican officials called those findings "premature," even as the FDA issued an advisory stating that a contaminated jalapeño pepper originated in Mexico.

Mexico's National Sanitation and Farm Food Quality Service director Enrique Sanchez told The Associated Press last week that Mexico sent a letter to the United States on Friday "expressing our concern and our most forceful complaint against this decision."

According to AP reports, Sanchez said the FDA "has no scientific proof to make a decision that will harm Mexico enormously."

Earlier, the FDA announced it had discovered salmonella on a jalapeño imported from Mexico at The Agricola Zarigosa produce distribution center in McAllen, Texas.

The FDA said traceback studies of food eaten by victims who became sick indicate the contaminated jalapeño pepper originated in Mexico.

The agency concluded the distribution center was not the source of the outbreak because peppers from a number of clusters never passed through there, said Dr. David Acheson, the agency's director of food safety.

To date, all traceback studies have led to Mexico and peppers grown in the United States have not been connected to the outbreak, he said.

Peppers grown in the United States have not been connected to the outbreak that has sickened more than 1,000 people since April, said FDA spokesman Michael Herndon.

Initially, tomatoes seemed the most likely source of the outbreak. The FDA told consumers to avoid certain raw tomatoes on June 7, prompting grocery chains and some restaurants nationwide to stop offering them.

The agency subsequently lifted that ban, determining that tomatoes currently in fields and stores are safe. Learn about the differences between salmonella and E. coli »

The FDA now advises consumers to avoid raw jalapeño peppers grown in Mexico and any foods containing raw jalapeño peppers grown in Mexico.

Only immune-compromised people, the elderly and infants should avoid raw serrano peppers from Mexico, Herndon said.

Cooked or pickled peppers from cans or jars are not part of the warning. Learn how to keep your food safe »

So far, 1,294 people infected with the same type of Salmonella Saintpaul have been identified in 43 states, the District of Columbia and Canada, according to the U.S. Centers for Disease Control and Prevention.

At least 242 have been hospitalized.

This particular Salmonella Saintpaul fingerprint is normally seen in only 25 cases a year, said Ian Williams, chief of the CDC's Outbreak Net Team.