Stores on Nov. 20 removed all products containing romaine. ( Ashley Nickle )

In a Thanksgiving Day tweet, Food and Drug Administration Commissioner Scott Gottlieb said the agency believes the source of the E. coli-tainted romaine responsible for a nationwide ban on the lettuce is California.

Gottlieb had reported a day earlier that it was “likely” the lettuce had been grown in California, but his Thanksgiving message seemed more certain.

#FDA continues to investigate source of e.coli outbreak traced to Romaine. We believe it's related to lettuce harvested from California. We hope to have more information by Monday isolating the growing region. New crop will soon harvest from other regions,” according to the tweet.

Ten of the 32 U.S. cases are in California, but the rest are in 10 states east of the Mississippi River. Canadian health officials reported 18 cases there.

When the Centers for Disease Control and Prevention announced the outbreak Nov. 20, it called for all romaine to be removed from the supply chain, and for shippers to stop packing and distributing all romaine products, from whole heads, hearts, bagged salads and spring mix. Retailers were asked to pull all romaine from shelves.

The United Fresh Produce Association on Nov. 20 advised companies to quickly comply and urged anyone contacted by regulatory agencies investigating the origin to help and make shipping records available. But it had also requested federal agencies to consider narrowing down the regions where romaine had been harvesting from when illnesses were reported, roughly mid- to late November.

This is an extremely broad warning to consumers to not eat any type of romaine from any growing region,” according to a member alert from United Fresh. “Despite our urging that industry could clearly identify some sources of romaine coming onto the market as not related to the outbreak, CDC and FDA also are requesting the voluntarily withdrawal of romaine lettuce before it enters commerce."

According to United Fresh, Yuma, Ariz., romaine had not commenced when the first illnesses were reported in mid-October.

Western Growers also asked regulators to use harvest dates and illness onset dates to target the possible growing region.

“ … It is important to acknowledge that a number of regions in current production were not harvesting or shipping romaine at the onset of the outbreak and, consequently, could not be the source of the specific E. coli strain identified in the illnesses,” according to a Western Growers statement on Nov. 21. “In light of this evidence, we urge the government’s health agencies to work with stakeholders to quickly narrow the scope of the investigation, and to remove these regions from the comprehensive advisory as soon as the safety of the public can be ensured.”