This brings the total number of people who have reported infections from 15 states to 52.
Nineteen people have been hospitalised, including two people who developed hemolytic uremic syndrome, a type of kidney failure, the agency said. No deaths have been reported.
Evidence collected from the United States and Canada indicates that romaine lettuce harvested from the central coastal growing regions of northern and central California is a likely source of the outbreak, the agency added.
The agency also said it continues to advise against the consumption, and sale of romaine lettuce harvested from these regions.
The blanket nationwide U.S. public health warning for consumers, precipitated by an E.coli outbreak, was first issued last month by the U.S. Food and Drug Administration and the U.S. Centers for Disease Control and Prevention.