Thanksgiving is less than a week away, and federal health officials are still trying to identify the source of a salmonella illness outbreak linked to raw turkey products that has spread to 35 states and sickened 164 people.

The outbreak, which started a year ago, has sent 63 people to the hospital. One person in California has died. The salmonella strain has been found in raw turkey pet food in Minnesota, raw turkey products collected from people’s homes and live turkeys from several states, indicating the bacteria is widespread in the industry.

No common supplier has been identified, according to officials at the Agriculture Department and the Centers for Disease Control and Prevention, so consumer groups are asking for the names of the brands and the slaughterhouses and processing facilities where the outbreak strain has been found in samples.

“When you are coming up on a major holiday where you know consumers are likely to consume the food in question, we think the agency has an obligation to give consumers whatever information it does have,” said Laura MacCleery, policy director at the Center for Science in the Public Interest.

Health officials say the investigation has been complex because the strain has been identified in a wide range of products, and investigators have been interviewing sick people to trace it back to a single source. Without a source or supplier of the product or products that are making people sick, officials say the best advice for consumers is to handle raw turkey carefully – including washing their hands, cutting boards and other utensils after touching raw turkey – and to cook it thoroughly to prevent illness.

Salmonella causes more than 1 million illnesses every year, and food is the main source. Symptoms of salmonella infection include diarrhea, abdominal pain and fever. Symptoms usually begin within 12 to 96 hours after exposure, but they can begin as long as one or two weeks after a person is exposed to the bacteria. Most people infected with the bacteria recover within a week without treatment, but serious cases may require hospitalizations for invasive infections, such as meningitis and bloodstream infections.

Antibiotics may be used to treat more serious infections. In the current outbreak, samples of the bacteria were found to have varying levels of antibiotic resistance, according to the CDC. But most of the commonly used antibiotics should be effective, and CDC’s advice to clinicians is the same for this outbreak. Health-care providers should order specific tests for patients to make sure they are treated with the appropriate antibiotic.

In recent days, consumer groups have called on the USDA to identify the brands of turkey that have been linked to the outbreak and to disclose the names of the companies that operate the 22 slaughterhouses and seven processing facilities where the bacterial strain has been found. The strain in those samples is closely related to the strain found in sick people, providing more evidence that people were sickened from preparing raw turkey products, according to the CDC.

In a letter sent Thursday to Agriculture Secretary Sonny Perdue, a coalition of consumer groups called on the USDA to release the information.

“Providing this information would allow some consumers to avoid turkey products more likely to be contaminated with dangerous salmonella,” said the Safe Food Coalition, which includes several consumer groups including Consumer Reports and the Consumer Federation of America.

But the USDA’s Food Safety and Inspection Service, which is monitoring the outbreak, said in a statement that it would be “grossly irresponsible and reckless” to identify the brands or name the companies that operate the facilities “when a link from an establishment to an illness has not been made.”

The National Turkey Federation said its producers follow “comprehensive pathogen control programs” from the hatchery through processing that are designed to reduce bacteria. At processing plants, the focus is on areas where contamination is most likely to occur. Producers use food-grade rinses that kill or reduce the growth of bacteria, organic sprays to clean the turkeys and inhibit bacteria, and USDA inspectors are “continuously present” in every turkey facility to monitor food safety, the industry group said.

According to the federation, nearly 88 percent of Americans surveyed by the federation eat turkey at Thanksgiving. In 2017, they estimated 44 million birds were eaten on Thanksgiving, 22 million at Christmas and 19 million at Easter.

Since the investigation began in February, state public health officials have been interviewing patients, looking at grocery receipts and shopper card data, and testing any turkey products that they had in their homes to search for a common supplier.

The states with the largest number of cases are Minnesota (17), Illinois (16), California (13), New York (12) and Texas (11). In February, a pet food manufacturer in Minneapolis, Raw for Paws, recalled about 4,000 pounds of its ground turkey pet food after two children became sick after the pet food was fed to pets in their home. One child’s infection resulted in a painful and serious bone infection, and the child was hospitalized, according to health department officials.