Michigan agriculture officials had previously named the supplier of the lettuce as Aunt Mid's Produce of Detroit but had not identified where the lettuce was grown.
The outbreak, involving bagged, industrial-sized packages of iceberg lettuce sold through wholesale venues to restaurants and institutions, sickened students at Michigan State University and the University of Michigan, and inmates at Lenawee County Jail before spreading to metro Detroit.
The Detroit Free Press reported late Thursday that Michigan agriculture officials had confirmed the state of origin, although a region wasn't specified.
Several questions remain to be answered, including in which part of California the lettuce originated.
Bob Perkins, executive director of the Monterey County Farm Bureau, said September is peak season for Salinas Valley lettuce growers.
"That's when our growers are very busy," said Perkins. "If it's California bagged lettuce, there's a real probability that it will be tied to our area, or to somebody that we know."
Even if it turns out that the lettuce was grown outside the Salinas Valley, he expects the implications could weigh heavily on a leafy green industry still reeling from the 2006 E. coli outbreak in spinach that sickened more than 200 and left three people dead.
Consumers from California may be familiar enough with the state's geography to differentiate the San Joaquin Valley or Imperial Valley from Salinas Valley, but Perkins thinks that's not likely the case for someone living outside the state.
"For anybody outside of California," he said, "what they're going to remember is California."
As of late Thursday, the news that California had been identified hadn't yet traveled through the local industry.
Perkins said the other big question will be whether health officials will be able to suggest a possible cause for how the bacteria was introduced.
"Everybody's going to want to know as much as possible about the potential causes, because everybody is doing pretty much everything they can to prevent outbreaks," he said.
For consumers reading about food safety outbreaks, he suspects it's hard to know what choices to make. And confusion doesn't help the industry sell its product, he said.
"Just talking about California certainly affects consumer confidence," he said.
Likewise, Dennis Donohue, president of the Grower-Shipper Association of the Central Coast, said the determination that California is the source of the lettuce is only one part of a complete picture.
"How was the product handled by the processor? How was the product handled by the product's consumers? How was it consumed?" he said.
As a founding member of the California Leafy Greens Handler Marketing Agreement, Donohue said he was proud of the efforts grower-shippers have made to ensure food safety.
"Obviously we would hope that the source would not be identified with that membership," he said. "But no one has ever said it would be a zero-incident world."
But wherever in California the lettuce turns out to have been grown, Donohue said, it will have some impact on consumer trust.
"Consumers, in terms of confidence levels, they tend not to split hairs. So the strongest link is affected by the weakest link," said Donohue. "This is an issue that has affected our industry, if nothing else, in costs and practices, and we're going to have to be eternally vigilant."
Aunt Mid's Produce of Detroit was identified as one of the Michigan suppliers. The company immediately stopped its lettuce distribution, said Chief Executive Officer Philip Riggio, and had its supply and processing facilities tested by outside experts. The tests found no evidence of contamination.
The Michigan Department of Agriculture also tested Aunt Mid's lettuce, with no findings of E. coli, said Jennifer Holton, MDA spokeswoman.
Holton said Aunt Mid's will be able to resume operations soon and the investigation is ongoing in cooperation with California food and safety officials.