(UPDATED COVERAGE, Sept. 16, 4:33 p.m.) Shredded lettuce is taking the blame for sickening 124 people across the U.S., primarily in Western states, and 12 in Canada with salmonella this summer.

Bill Keene, senior epidemiologist for The Oregon Public Health Division, said shredded lettuce, possibly from quick-service restaurant locations, is the likely cause of a Salmonella typhimurium outbreak that started mid-July, peaked in August, and tapered off later that month.

Keene has been involved in the epidemiological traceback for weeks, he said, and recently saw the investigation mostly handed over to the Food and Drug Administration and state departments of agriculture, as shredded lettuce seems to be a fairly common denominator for affected people.

"FDA launched its involvement around the first of September," said Sebastian Cianci, spokesman for the FDA. "So far we have not been able to identify a vehicle of transmission, so we don't know a vehicle yet, but we do believe it's food related."

There were no lettuce recalls associated with this outbreak, which makes the investigation difficult. Any contaminated product would have been consumed by now, Keene said, so traceback cannot be confirmed by testing.

Consumer reports of food consumption are being used as primary evidence, along with invoices and shipping and receiving records from restaurants, distributors and shippers.

Cianci said the outlet for the unconfirmed source of contamination seems to be fast food restaurants, although not necessarily chain restaurants.

"People reported eating at one or more fast food restaurants, often multiple restaurants," Cianci said. "But it doesn't seem to be one restaurant chain or one type of food served. And it does not seem to be associated with food sold at grocery stores."

There are still a few loose ends to tie up in epidemiological testing, Keene said.

“The epidemiological evidence can be confirmatory, and that’s what’s still in progress,” Keene said. “There still is work to be done to rule out other possibilities. But it’s fair to say that most people say this is the one.”

Although Keene and a portion of his colleagues are convinced, authorities hesitate to convict shredded lettuce, partly in light of false allegations against tomatoes during the Salmonella Saintpaul outbreak in 2008.

“I don’t think anyone’s still arguing it’s something else, but some say the evidence isn’t quite there,” Keene said. “People are understandably and appropriately reluctant to cross the ‘T’s and dot the ‘I’s before all the evidence is in.”

It’s also because the strain of salmonella, Salmonella typhimurium, is common, so baseline cases of the disease were being reported before and throughout this investigation. The molecular subtyping pattern is also very common.

“So we don’t have a fabulous marker to tell if a person is clearly part of this outbreak,” Keene said. “That has slowed and hindered this investigation.”

Cases were reported across the U.S. and in Canada, although most were in the Western part of the country. Keene could not confirm whether any one state or any one shipper is suspected, but he did say at least one obvious lettuce growing state is involved.

“California is certainly a state where a lot of lettuce is growing, so it’s very interested. It’s one of the states involved,” Keene said Sept. 15. “At this point, nobody knows how the original contamination occurred. Even if it was shredded lettuce, there’s not a candidate field as of today.”

Keene said because the lettuce was shredded, there are also possibilities in the processing steps of the supply chain.

One thing that’s clear, Keene said, is that the outbreak is over. However, the CDC still classifies the outbreak as ongoing because of the baseline cases of this type of salmonella that will carry over. That makes it difficult to see a clear end in cases.

“So there’s no urgency now to do anything to protect consumers, but we’re still interested in finding out what caused this, whether it’s a deficient practice or just one more lettuce outbreak…” Keene said.

The Centers for Disease Control and Prevention and the Food and Drug Administration continue to investigate the cause of the outbreak, along with state departments of agriculture and public health.

The disease did not cause any fatalities, although two people were hospitalized, according to media reports.