WASHINGTON (AP) — The first sickness was in March and the first signs of a salmonella outbreak appeared in May. Two months later, investigators linked the outbreak to ground turkey and a Cargill meat processing plant in Arkansas.

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On Wednesday, almost five months after the first illness, the Agriculture Department asked Minnesota-based Cargill to recall 36 million pounds of ground turkey, saying the meat was linked to a death in California and at least 77 illnesses.

Tracking down the source of an illness is a difficult, complicated business, and federal officials defended the months-long process Thursday by saying they wanted to be absolutely sure before they asked Cargill to initiate the third-largest meat recall in history.

“There was an aggressive and thorough investigation that came together over time to paint one picture of this outbreak,” said Dr. Christopher Braden, an epidemiologist at the federal Centers for Disease Control and Prevention, which investigated the outbreak with the Agriculture Department.

Investigations are often delayed because victims can’t remember what they ate or aren’t cooperative, says Caroline Smith DeWaal, food safety director for the Center for Science in the Public Interest. But DeWaal says she believes the government should have told the company — and the public — about the possible outbreak much sooner.

“Clearly this kind of delay in an outbreak situation is one that puts the public health at risk,” DeWaal said.

Braden and David Goldman of the USDA’s Food Safety and Inspection Service said at a briefing Thursday that authorities weren’t sure about the source of the outbreak until July, when they identified it through shopper’s card information from victims who had purchased the Cargill turkey, and a leftover sample of turkey from a victim’s home. Previous evidence from the investigation had pointed not just to Cargill, but other companies as well, they said.

Less conclusive evidence had pointed toward the Cargill plant in Springdale, Ark., much earlier, however. Samples of Cargill ground turkey purchased at four retail locations as part of routine testing between March 7 and June 27 showed contamination with the same strain of salmonella, though those samples were not specifically linked to the illnesses. Routine salmonella testing in 2010 also showed the pathogen at the plant.

Part of the difficulty for investigators is that USDA rules make it harder to investigate and recall salmonella-tainted poultry. Because salmonella is so common in poultry, it is not illegal for meat to be tainted with the pathogen. General guidance to consumers is to cook ground turkey to 165 degrees and to handle it properly before it is cooked. If it is cooked and handled properly, it is safe.

Braden and Goldman warned consumers to check their freezers for the contaminated products because frozen ground turkey has a long shelf life. The recall involves fresh and frozen ground turkey products produced at the company’s Springdale, Ark., plant from Feb. 20 through Aug. 2.

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All of the packages recalled include the code “Est. P-963,” according to Cargill, though packages were labeled under many different brands. Many of the recalled meats are under the label Honeysuckle White. Other brands include Riverside Ground Turkey, Natural Lean Ground Turkey, Fit & Active Lean Ground Turkey and Shady Brook Farms Ground Turkey Burgers.

The recall also includes some ground turkey products packaged under the HEB, Safeway, Kroger, Randall’s, Tom Thumb and Giant Eagle grocery store brands or sold at those stores.

The recall also includes some ground turkey that isn’t labeled at all and some that went to food service establishments, according to Cargill.

Seventy seven illnesses in the outbreak — an increase of one from the 76 illnesses reported earlier in the week — have been reported in 26 states coast to coast. A chart on the CDC’s website shows cases have occurred every month since early March, with spikes in May and early June. The latest reported cases were in July, although the CDC said some recent cases may not have been reported yet.

The CDC said the strain is resistant to many commonly prescribed antibiotics, which can make treatment more difficult. Braden said that at least 22 people were hospitalized, which is more than would be expected. One possible reason for that is the antibiotic resistance, Braden said, though the strain is responsive to some antibiotics.

USDA’s Goldman said the department is continuing to investigate and analyze samples from the plant itself. Outside the plant Thursday, a tractor-trailer carrying hundreds of turkeys pulled into a shed. Fans and machines spraying mist kept the turkeys cool as government inspectors worked inside. An Associated Press photographer was not allowed inside the plant.

In announcing the recall, Cargill officials said all ground turkey production has been suspended at the Springdale plant until the company is able to determine the source of the contamination.

“Given our concern for what has happened, and our desire to do what is right for our consumers and customers, we are voluntarily removing our ground turkey products from the marketplace,” said Steve Willardsen, president of Cargill’s turkey processing business.

The CDC estimates that 50 million Americans each year get sick from food poisoning, including about 3,000 who die. Salmonella causes most of these cases, and federal health officials say they’ve made virtually no progress against it.

The most common symptoms of salmonella are diarrhea, abdominal cramps and fever within eight to 72 hours of eating a contaminated product. It can be life-threatening to some with weakened immune systems.

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