WASHINGTON — The Food and Drug Administration’s (F.D.A.) traceback investigation of an E. coli O157:H7 outbreak that has sickened at least 65 people indicates the product may have been sourced from growing regions in central California. Romaine lettuce harvested from locations outside of California do not appear to be related to the current outbreak.

“There is no recommendation for consumers or retailers to avoid using romaine lettuce that is certain to have been harvested from areas outside of the Central Coast growing regions of northern and central California,” the F.D.A. said. “For example, romaine lettuce harvested from areas that include, but are not limited to the desert growing region near Yuma, the California desert growing region near Imperial county and Riverside county, the state of Florida, and Mexico, does not appear to be related to the current outbreak. Additionally, there is no evidence hydroponically- and greenhouse-grown romaine is related to the current outbreak.”

To aid consumers in identifying the growing region of romaine lettuce, growers, packers and marketers have agreed to begin labeling product to identify the region of the country it was sourced.

“Knowing the growing origin of produce will continue to play an important role in allowing consumers to avoid contaminated products and facilitating market withdrawals and tracebacks,” said Scott Gottlieb, M.D., commissioner of the F.D.A. “To this end, the F.D.A. recently participated in discussions with the major producers and distributors of romaine lettuce in the U.S. and with the major trade associations representing the produce industry regarding product labeling and dating to assure consumers that any romaine lettuce that will come onto the market is not associated with the current outbreak of E. coli O157:H7.”

Product labeling will include the origin of the romaine based on harvest region and the date it was harvested. The leafy greens industry also has agreed to create a task force to find solutions for long-term labeling of romaine lettuce and other leafy greens, according to the F.D.A.

The Produce Marketing Association (P.M.A.), Newark, Del., expressed its support for the labeling initiative.

“We recognize that voluntary source labeling is not true traceability, but it is a mechanism to ensure consumers have access to romaine that was not implicated in the outbreak,” the P.M.A. said. “A standardized approach to source labeling is new to the industry and we expect situation-specific questions that were not anticipated. All involved, including F.D.A., recognize that we will need to gain real-world experience as companies develop the most workable approach in different situations.”