UPDATE: WASINGTON (AP) -- Federal health officials say the salmonella strain linked to a nationwide outbreak has been found in irrigation water and a serrano pepper at a Mexican farm.

Dr. David Acheson, the Food and Drug Administration's food safety chief, is calling the finding a key breakthrough in the case.

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

Acheson and other officials were grilled Wednesday 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.

ORIGINAL REPORT: WASHINGTON -- Government inspectors finally have a big clue in the nationwide salmonella outbreak: They found the same bacteria on a single Mexican-grown jalapeno pepper handled by a small Texas produce shipper.

But Monday's discovery doesn't solve the mystery: Authorities don't know where the pepper became tainted - on the farm, or in the plant in McAllen, Texas, or at some stop in between.

Nor are they saying the tainted pepper exonerates tomatoes sold earlier in the spring that consumers until last week had been told were the prime suspect.

Still, "this genetic match is a very important break in the case," said Dr. David Acheson, the Food and Drug Administration's food safety chief.

But for now, the government is advising consumers to avoid eating any fresh jalapenos, or products made with them, such as fresh salsa. Tomatoes now on the market are considered safe to eat, health officials have said since last week.

The Texas plant, Agricola Zaragosa, has agreed to recall its fresh jalapenos, although the FDA wouldn't say how widely its produce was distributed. It is not considered a major processor, and it's unclear how far into the United States peppers traveling through this small stop near the Mexican border would have traveled, something the FDA still is working to determine.

It's also not clear if the produce distributor ever handled tomatoes. No other produce currently in the plant has tested positive for salmonella, Acheson said.

"I recognize there is a need to narrow this as soon as possible," Acheson added - as parts of the country are entering prime hot pepper season.

A person who answered the phone at Agricola Zaragosa said no one was available to comment immediately.

With 1,251 confirmed cases in 43 states - and a few among Canadian travelers to the U.S. - the outbreak isn't over yet, said Dr. Robert Tauxe of the Centers for Disease Control and Prevention. But the CDC said last week that it appeared to be slowing, and indeed has confirmed just 14 additional cases since then. The latest that someone fell ill was July 4.

(Copyright 2008 by The Associated Press. All Rights Reserved.)