WASHINGTON (AP) -- The government said Thursday that the salmonella outbreak that sickened at least 1,440 people appears to be over, but its ultimate source may never be known, partly because of shortcomings in the nation's food safety system.

The Centers for Disease Control and Prevention and the Food and Drug Administration said they found strong evidence to implicate jalapeno and serrano peppers, and a farm in Mexico, in the largest outbreak of foodborne illness in a decade. Investigators were unable to clear domestic and imported tomatoes, however, although the evidence against tomatoes is weaker.

The FDA also lifted its warning that consumers avoid eating jalapeno and serrano peppers from Mexico. But officials pointedly said that doesn't guarantee another such outbreak can be prevented.

"None of us can provide a cast-iron guarantee that salmonella Saintpaul will not re-emerge," said Dr. David Acheson, the FDA's food safety chief. "We have not identified the total source of this."

FDA and CDC officials said a number of steps are needed to improve the safety of fresh produce, even as the government and the medical community are urging consumers to eat more fruits and vegetables for better nutrition.

Among those measures: Standard procedures and more funding to allow state laboratories to test samples of suspected pathogens more rapidly. Congressional action to give the FDA authority to impose produce safety regulations. And industry action to develop a faster system for tracing back to the farm any produce items suspected in an outbreak.

The CDC said the outbreak began in late April, and that by early August the number of new cases had fallen to levels that would be considered normal. Most victims got sick during May and June. And there have been no new restaurant clusters of cases since early July. That "is an important indication that this particular outbreak is over," said Dr. Robert Tauxe, deputy director of CDC's foodborne illness branch.

Texas was the hardest-hit state, accounting for nearly 40 percent of the all confirmed cases. People were sickened in 43 states and Washington, D.C.

The joint investigation by CDC and the FDA found strong evidence that jalapeno peppers were a major carrier of the outbreak bacteria, and that serrano peppers were also a carrier. It was the first time that jalapenos were implicated in such an outbreak.

The salmonella strain was traced back to a jalapeno pepper at a produce distribution center in Texas that received peppers from Mexico. But FDA investigators struck out when they performed tests at the farm in Mexico where they believed the pepper had been grown.

Instead, they found the bacteria on another Mexican farm about 100 miles away from the first. The outbreak strain was isolated from water in a pond used for irrigation and from a sample of serrano peppers. Acheson said it is not completely clear that the second farm was the source of the outbreak.

Both farms provided produce to a common packing facility in Mexico, which shipped to the United States. That raises the possibility that contamination could have occurred during packing and shipping.

Consumers around the country first heard about the problem June 7, when the FDA issued a broad warning against eating various kinds of tomatoes.

Yet the extensive probe found not a single contaminated tomato. Still, investigators said they cannot rule tomatoes out as a carrier, particularly early in the outbreak. Interviews with patients who got sick suggested a strong link to tomatoes, which had been implicated in previous salmonella outbreaks.

"We continue to believe that association could reflect real contamination early on," said CDC's Tauxe. But he acknowledged the evidence is weaker when it comes to tomatoes.

"It is information that is more restricted in time and does not have confirmatory laboratory findings behind it," he said.

As the focus shifted to peppers, the U.S. tomato industry complained that the government had unfairly singled it out based on flimsy evidence, leading to an estimated $250 million in losses.

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