The US nationwide salmonella outbreak may have passed its peak but officials from the US Food and Drug Administration (FDA) have revealed that they do not yet understand how the few sources they have identified led to such a major food scare.

According to the FDA, laboratory testing has confirmed that both a sample of serrano pepper and a sample of irrigation water collected on a farm in Tamaulipas, Mexico contain salmonella Saintpaul with the same genetic fingerprint as the strain of bacteria that is causing the current outbreak in the US.

A prior positive sample of a Mexico-sourced jalapeño was obtained during an FDA inspection at a produce distribution centre in McAllen, Texas.

The FDA’s advisory to avoid eating raw jalapeño and serrano peppers, and foods that contain them, applies only to these types of peppers grown, harvested or packed in Mexico, the agency said in a statement.

However, the FDA’s head of food safety, Dr David Acheson, told Associated Press that the outbreak is not yet finished, although no new cases of the illness have been reported since 24 July. The current total of affected cases stands at 1,423 people.

Dr Acheson said the agency had now expanded its investigation to include a range of Mexican producers and said traces of salmonella, although not of the same strain, had been found at 16 farms, including those of producers of basil and cilantro as well as peppers.