ALLENDE, Mexico -- At the end of a dirt road in northern Mexico, the conveyer belts processing hundreds of tons of vegetables a year for U.S. and Mexican markets are open to the elements, protected only by a corrugated metal roof.
The U.S. Food and Drug Administration suspects this packing plant, its warehouse in McAllen, Texas, and a farm in Mexico are among the sources of the United States' largest outbreak of food-borne illness in a decade, which infected at least 1,440 people with a rare form of salmonella.
A plant manager confirmed to The Associated Press that workers handling chili peppers aren't required to separate them according to the sanitary conditions in which they were grown, offering a possible explanation for how such a rare strain of salmonella could have caused such a large outbreak.
The AP has found that while some Mexican producers grow fruits and vegetables under strict sanitary conditions for export to the U.S., many don't -- and they can still send their produce across the border easily.
Neither the U.S. nor the Mexican governments impose any safety requirements on farms and processing plants. That includes those using unsanitary conditions -- like those at Agricola Zaragoza -- and brokers or packing plants that mix export-grade fruits and vegetables with lower-quality produce.
In fact, the only thing a Mexican company needs to do to sell produce to the United States is to register online.
Some Mexican farms and processing plants have high standards of sanitation -- and get private companies to certify those standards -- so they can sell to U.S. supermarket chains that wouldn't buy from uncertified ones.
But there is no public list of the chains that require sanitary practices, meaning there's no way to know whether the fruit and vegetables in any particular store are certified.
The only U.S. government enforcement consists of 625 FDA inspectors who conduct spot checks of both U.S. and foreign produce, reviewing less than 1 percent of all imports. Beyond that, it is entirely up to the supermarkets and restaurants to police their produce.
The best Mexican producers grow crops in fenced-off fields, irrigate them with fresh water and pack them in spotless plants where workers dress in protective gear from head to toe. But there are still plenty of farms with unfenced fields where wildlife can roam freely, and which use untreated water -- sometimes laced with sewage.
Salmonella can lurk on the skin of produce or penetrate inside. Cooking kills it, but washing raw produce doesn't always eliminate it, which is why safety experts stress preventing contamination.
Agricola Zaragoza is one of the uncertified plants, manager Emilio Garcia told the AP. He said the packing plant washes produce from both certified and uncertified producers, opening up the possibility for contamination. He refused to give details about his suppliers.
The FDA suspects Mexican jalapeno and serrano chilies processed at Agricola Zaragoza caused the latest outbreak, though it also thinks tomatoes could have played a role. It concedes the ultimate source may never be known.
Cesar Fragoso, president of Mexico's Chili Peppers Growers Association, said most Mexican pepper farms sell their crops to distributors without knowing what country they are bound for. Because of that, he said, few bother to get certification.
In addition, lots of produce passes from distributor to distributor before reaching its final destination, increasing the potential for contamination and making tracing outbreaks much more difficult. Former FDA official William Hubbard said only 10 percent of outbreaks are ever completely resolved.
"It is very common for distributors to receive products from numerous sources, numerous farms and in some cases multiple countries," Hubbard said. "That's just the way produce moves."
In the latest contamination case, the U.S. government traced the suspect jalapenos to two farms in the state of Tamaulipas. Both shipped through Agricola Zaragoza in neighboring Nuevo Leon state. Agricola Zaragoza shipped the peppers to its warehouse in McAllen, Texas, where the FDA found the first contaminated jalapeno.
Though usually smaller in scale, such outbreaks are relatively common -- at least 3,000 between 1990 and 2006 from FDA-regulated foods, according to the Center for Science in the Public Interest, a nutrition and food safety advocacy group. Those numbers include fruits, vegetables and seafood, and contamination both in the U.S. and abroad.
The cases include a 2004 hepatitis outbreak linked to Mexican green onions that killed four people and sickened 650 in Pennsylvania, and a 2006 nationwide E. coli outbreak that infected about 300 people and killed three and was traced to tainted spinach from California.
The U.S. Senate is considering a bill that would require the FDA to issue regulations for ensuring safer fresh produce. In Mexico, a federal produce safety law was passed in 1994 but analysts say it is rarely enforced. Mexico's Agriculture Department did not respond to a request for an interview.
Kathy Means, a vice president for the U.S. Produce Marketing Associations, said food safety is in the hands of the food industry, with most major produce buyers requiring both U.S. and foreign food producers to have third-party audit programs. However, Means said, not all buyers follow the same rules.
"It's not government-regulated, so it's up to the company to require it," she said.
At Alfonso Alvarez's fenced-off 15-acre farm in Jalisco state, tomatoes are grown in greenhouses and irrigated with water from a deep well. Workers wear hair nets, gloves and aprons, and signs require them to wash their hands after going to the bathroom.
Alvarez sells its crop to a Canadian company that imports to the U.S. and Canada and has required his farm be certified by a U.S. private company.
"Those of us who want to enter the U.S. market and position our brand know we must meet all those standards, because we also know it will be a profitable business in the long run," Alvarez said.
He and other Mexican farmers with sanitary farms want the United States to set up a certification program that covers both growers and packing plants.
"Those who grow in open fields will ruin it for those who produce in greenhouses," Alvarez said, "and that's not fair."
Associated Press writers Mark Walsh reported this story from Allende and Olga R. Rodriguez from Mexico City and Zapoltitic, Mexico. AP medical writer Lauran Neergaard in Washington contributed to this report.