WASHINGTON – The nationwide salmonella outbreak is finally winding down but federal health officials can't yet say how the few tainted Mexican peppers they've found could explain such widespread illness.

The outbreak isn't considered over yet, Food and Drug Administration food safety chief Dr. David Acheson cautioned Friday. The outbreak strain has been confirmed in 1,423 patients, with the latest known illness beginning July 24.

The FDA is focusing its probe on some farms in Mexico where a handful of jalapeno and serrano peppers, and some irrigation water, tainted with the outbreak strain of salmonella were traced. At least one of the farms also grew tomatoes – the initial suspect – as well as peppers. And two of them sent produce to a common packing facility, raising the prospect that contamination there could have spread to a much higher volume of food.

The FDA said Friday it is still working with Mexican authorities to determine exactly what happened in that packing facility.

And the agency has expanded testing of certain Mexican produce, uncovering more cases of salmonella contamination – just not the same strain that caused this particular outbreak – in jalapenos, basil and cilantro. While Acheson wouldn't say how much salmonella is being found, the agency has put a dozen Mexican growers or distributors on its “import alert” list for tougher border screening this month alone.