U.S. officials said Monday they still haven't pinpointed the source of a nationwide outbreak of dangerous E. coli infections from romaine lettuce.

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WASHINGTON — Eight sick prisoners in Nome, Alaska, have provided a clue to authorities about the origin of a nationwide outbreak of dangerous E. coli infections from romaine lettuce, but U.S. officials said Monday they still haven’t pinpointed the source of the contamination.

Instead, the Food and Drug Administration and the Centers for Disease Control and Prevention are sticking with a broad warning to consumers, telling them to throw away romaine, in any form, that comes from the Yuma, Arizona, growing region, and to avoid eating romaine of unknown origin. Most of the romaine sold in the United States during the winter is grown in the Yuma region.

This is dangerous strain of E.coli, known as 0157:H7. It produces a Shiga toxin that can enter a person’s bloodstream and wreak havoc on kidney function. Symptoms of infection include vomiting, painful cramps and diarrhea that is often bloody.

Outbreak investigations often take weeks because food in the United States is handled and processed many times as it is distributed nationwide. Typically, the contamination comes from the feces of an animal coming into contact with the produce. Investigators are searching the Yuma area and doing field tests trying to find where the problem originated.

The Nome case, revealed late last week, could potentially be a break in the investigation, because all the prisoners at the Anvil Mountain Correctional Center ate lettuce that came from a single supplier. An official in Alaska, Jeremy Ayers, a section manager in the state’s food safety program, reiterated Monday that the lettuce had been traced to a farm in Yuma.

Using purchase orders, state officials traced the romaine to one lettuce supplier for the jail. That supplier received its lettuce from one packer in California, which got its romaine from one farm in Yuma, he said. “We just kept doing that until we arrived at the farm,” Ayers said.

But FDA spokesman Peter Cassell, on Monday, said that, although the Alaska information has been helpful, it has not solved the mystery. No single farm has been identified as the location of the contamination nationwide, he said.

“FDA’s investigation is still ongoing. What they have is not conclusive,” Cassell said, referring to the Alaska probe. It’s not known publicly whether the farm in Yuma and the packer in California also sent romaine to other suppliers. “We are still working on the traceback. We have not traced it back to a particular farm or supplier,” he said.

On Twitter on Monday, FDA Commissioner Scott Gottlieb said no specific grower, supplier, distributor or brand has been identified at this time. But he noted that romaine has a short shelf life, and the winter growing season is ending in Yuma. “It’s likely that any romaine sold now is from California, not AZ,” he tweeted. “Consumers should continue asking grocers and restaurants to make sure their romaine isn’t from Yuma.”

The Yuma region grows most of the romaine sold in the United States winter through March, before it becomes too hot in southwest Arizona, and lettuce production shifts to California, particularly the Central Valley and Salinas Valley.

The E.coli outbreak, which by last official count had sickened 53 people in 16 states, dates to March 13, with no known fatalities. That count included one of the Alaska prisoners. When all the Alaska inmates are included, the total number of people sickened is at least 60. So far, about two-thirds of the people sickened have been women, officials said, and the age range is 10 to 85.

The outbreak at the prison could be a key to figuring out the source. Salad was on the menu for dinner every day from March 26 through April 6, state officials said.

“The prison is a perfect control — they all ate the same things at the same time, and they ate romaine,” said Bill Marler, a food safety lawyer who says he has been involved in every major and minor E. coli outbreak in the last 15 years. “The chances that eight prisoners from Nome not being linked to the illnesses in the lower 48 is zero, given that they are a genetic match to the outbreak strain of E. coli,” Marler said. Marler said he is representing 28 individuals who have become sick, several of them children hospitalized with severe kidney failure. Seven or eight of his clients haven’t been interviewed by state health officials, he said, and thus are unlikely to be included in the official count of the extent of the outbreak.

Gary Weber, who until June served as prevention manager for the Food and Drug Administration’s Coordinated Outbreak Response and Evaluation network, told The Washington Post that he was puzzled that it has taken so long to identify the source of the contamination. With this many sick people, and with known clusters of illness — including at the Alaska prison and in restaurants in New Jersey — there should be enough information to trace the E.coli to a specific location rather than just a general geographic area, he said.

“This seems odd to me,” Weber said.

The CDC put out its first warning of an outbreak on April 10. It did not specify romaine lettuce as the food item involved.

Three days later the CDC put out an update stating that evidence “indicates that chopped romaine lettuce is the likely source of this outbreak.” Most people who became sick said they ordered salads in restaurants. The restaurants reported that they’d used bagged, chopped lettuce. “At this time, ill people are not reporting whole heads or hearts of romaine,” the CDC said on its website. It reported that “preliminary information” linked the lettuce to the Yuma growing region.

On April 20, the CDC changed its advisory to include a warning about whole-head lettuce. That came after the CDC learned of the Alaska cases. The prison had obtained whole-head lettuce that was consumed by the prisoners.

The CDC publishes an “epi curve” that shows that people began getting sick on March 13. Most of the cases documented involved people becoming sick between March 24 and March 28, with Pennsylvania leading the way with a dozen cases. But the CDC notes that cases since March 29 may not have been reported to state authorities. The most recent documented cases involved people who became sick on April 6.