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By Maggie Fox

California may be the source of romaine lettuce blamed in an outbreak of E. coli infections that has made 32 people sick in 11 states and Canada, the head of the Food and Drug Administration said Wednesday.

The FDA and the Centers for Disease Control and Prevention have cautioned people not to eat romaine lettuce while they investigate the outbreak. It’s very similar to an outbreak of E. coli that killed one person and made at least 25 people ill last year, FDA Commissioner Dr. Scott Gottlieb said. That outbreak was traced to leafy green vegetables but not to romaine lettuce specifically.

“The strain that caused that outbreak is very similar to the one that’s causing this outbreak and the timing is exactly the same. So it’s likely associated with end-of-season harvests in California, where most of the romaine that is currently on the market is from,” Gottlieb told NBC News.

“I think we are going to be in a position to isolate the region soon. There is some lettuce coming in from Mexico but most of what’s on the market is the result of end-of-the-season harvesting coming out of California right now.”

Gottlieb wrote in a tweet Friday that the contamination was "likely" from California and that the goal was to rid the market of possibly contaminated lettuce and replace it with romaine grown elsewhere. He added that the CDC is looking to take steps to improve labeling to make it clear where and when lettuce was harvested.

This is the second outbreak of E. coli linked to romaine lettuce this year. An outbreak this past spring killed five people and made 210 sick in 36 states. It was eventually traced to contaminated canal water in a farming area in Yuma, Arizona.

Until a specific farm or region can be identified, CDC and FDA say it’s safest just to tell people not to eat romaine lettuce and to tell restaurants and stores to throw it out. Other leafy greens appear to be safe.

Although this is the second romaine-linked outbreak this year, it is not clear whether romaine lettuce itself is more likely to be contaminated than other vegetables, said Dr. Laura Gieraltowski, leader of the foodborne outbreak response team at the CDC. “It’s something we’ve been asking,” she said.

“It could have to do with the way the lettuce is shaped. (A head of) romaine lettuce is kind of open and maybe contaminated water can get into it more than into a head of iceberg lettuce or cabbage that is in a ball.”

And like many other foods, romaine is often processed and repackaged before it ships out to grocery stores and restaurants.

“Is romaine somehow riskier? We don’t think so,” Gottlieb said. “We think it is probably related more to the way that it is packaged.” So pieces of a single contaminated head of romaine could be chopped up and spread through a number of packages.

What is known is where E. coli comes from. Like so many other bacteria that contaminate food, it comes from fecal matter. Wild animals may roam through fields, or irrigation water might flow from nearby pastures or feedlots where livestock is raised. Contamination can be further spread when produce is harvested and passes through machinery to clean, trim, chop and package it.

Tracking down the source is time consuming, which is why the FDA has not announced a mandatory recall yet, Gottlieb said. But he said some growers and distributors were planning to voluntarily recall their romaine products later Wednesday.

While it may seem that there have been more food recalls lately, both the FDA and CDC say that’s not because more food is being contaminated. “The food supply in the U.S. is one of the safest in the world,” Gieraltowski said.

Rather, FDA and CDC are better at finding ways to link the 48 million cases of food poisoning that occur each year.

“It’s not that there are more outbreaks. It’s that we are identifying more outbreaks,” Gottlieb said.

“A lot of what we identify now would have been random people showing up at their doctor’s office with gastrointestinal illness that we would never have associated with a common etiology (cause),” Gottlieb added.

Genetic fingerprinting has changed that.

“But now that we’re taking samples and looking at bacteria … we are seeing that it’s the same bug and that clues in researchers at the CDC that there’s one common element, that there’s one genetic fingerprinting, that it’s coming from the same source. We are seeing that it's the same bug."

The next step is speeding up the slow process of tracking the sources of the outbreaks using better technology to keep tabs on food as it moves from farms to distributors and to stores, Gottlieb said.

Contaminated food is an extremely common problem. The CDC estimates that germs in food make 48 million Americans sick every year — that’s one out of six people. About 128,000 are made sick enough to be hospitalized, and 3,000 die.