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.”

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.