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Taylor Farms of Mexico, a division of a California-based produce supplier whose greens go to national restaurants including Red Lobster and Olive Garden, shipped parasite-tainted salad mix that has sickened hundreds in Nebraska and Iowa, U.S. health officials said Friday.

Food and Drug Administration officials did not say whether the same bagged salad is tied to a cyclospora outbreak or outbreaks that have sickened at least 400 people in 16 U.S. states.

"The FDA traceback investigation found that illness clusters at restaurants were traced to a common supplier, Taylor Farms de Mexico, S. de R.L. de C.V.," the FDA said in a statement.

Those restaurants included Olive Garden and Red Lobster, operated by Darden restaurants, a spokesman for the Orlando, Fla., firm confirmed.

"The FDA's announcement today regarding Iowa and Nebraska is new information," said Rich Jeffers, communications director for Darden. "Nothing we have seen prior to this announcement gave us any reason to be concerned about the products we've received from this supplier."

FDA's investigation did not implicate salad mix packages sold in grocery stores, officials said. As a result of the probe, the agency will step up surveillance of leafy products exported to the U.S. from Mexico.

The Mexican plant is part of Taylor Farms, a Salinas, Calif.-based firm that supplies lettuce and cut vegetables to national restaurant chains and grocery stores. Taylor Farms has 11 processing plants in the U.S. and one in San Miguel, Mexico, according to the company website.

FDA officials, in conjunction with company leaders, will conduct an environmental assessment of the processing facility in Mexico to determine the probable cause of the outbreak. State officials had said the salad mix included romaine and iceberg lettuce, along with carrots and red cabbage. A 2011 inspection found no "notable issues," the FDA said.

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The firm's chairman and chief executive, Bruce Taylor, told NBC News in an email that the company has an extensive testing program in place in Mexico for both water sources and raw product.

"All our tests have been negative and we have no indication of the parasite in our product," Taylor said late Friday.

The contaminated salad mix is likely no longer in the food supply chain in Iowa and Nebraska, where at least 223 people have sickened, health officials emphasized. The last date people reportedly became ill in those states was July 2, they added. The typical shelf life for salad mix is about two weeks.

"Iowa and Nebraska health authorities have said this is not an ongoing outbreak and is no longer in the food supply in those states," said Jeffers, of Darden. "The health and safety of our guests is our top priority and it is completely safe to eat in our restaurants."

Taylor Farms has a history of recalling potentially contaminated leafy greens, including a February 2013 recall of baby spinach over fears it was tainted with Enterohemorrhagic E. coli, or EHEC, a particularly virulent bacterium that can cause severe infection and illness. The firm recalled bagged hearts of romaine in 2012 for listeria risk and bagged salad in 2011 over worries about salmonella contamination.

Food safety experts have criticized the investigation of the cyclospora outbreak, which began with two cases in Iowa on June 28, according to the Centers for Disease Control and Prevention. Most of the illnesses were reported from mid-June through early July.

Michael Osterholm, Minnesota’s former state epidemiologist who now heads the Center for Infectious Disease Research and Policy at the University of Minnesota, said the search for the source of the rare parasite took too long and wasn’t as thorough or targeted as it should have been.

“I think it’s really a mess,” Osterholm told NBC News. “To me it’s a situation where we need a major review.”

Osterholm said state investigators, including those in Iowa and Nebraska, which first tagged premixed salad as the source of the outbreak this week, didn’t conduct case-control studies that would have quickly isolated the cause.

But Dr. Patricia Quinlisk, the Iowa state epidemiologist, has defended her state’s response, saying that cyclospora is a difficult bug to detect and track because of its long incubation period and special testing requirements.

States that have reported illnesses include Iowa, Texas, Nebraska, Florida, Wisconsin, Georgia, Illinois, Arkansas, Kansas, Louisiana, Missouri, Connecticut, Minnesota, New Jersey, New York, and Ohio.

Cyclospora is a parasite excreted in human stool. Illnesses have been associated with contaminated water or food. It causes gastrointestinal symptoms including prolonged diarrhea, fatigue, loss of appetite, vomiting and other flu-like symptoms.

Cyclospora infections are rare in the U.S., but past outbreaks have been associated with contaminated fresh produce including fruit and herbs. Raspberries imported from Guatemala were responsible for a 1996 outbreak that sickened 1,465 people in the U.S. and Canada and also for a 1997 outbreak that made more than 1,000 people ill, CDC records show.

JoNel Aleccia is a senior health reporter at NBC News. Reach her on Twitter at @JoNel_Aleccia or send her an email.