ATLANTA — Public health and regulatory officials in the United States and Canada are investigating a multistate outbreak of E. coli infections linked to a Fresh Express chopped salad product.

The outbreak is caused by a different strain of E. coli than the current outbreak linked to romaine lettuce from California’s Salinas region, the Centers for Disease Control and Prevention (C.D.C.) said.

Eight people infected with the outbreak strain have been reported in three states. Three have been hospitalized. Health Canada reported 16 confirmed cases of illness linked to the outbreak and four hospitalizations.

Information collected by the C.D.C. indicates Fresh Express Sunflower Crisp chopped salad kits are a likely source of the outbreak. The investigation is ongoing to determine which ingredient in the salad was contaminated.

The Canadian Food Inspection Agency issued a food recall warning for the salad kits. U.S. health officials advised consumers, restaurants and retailers not to eat or sell the product.