At least six people have tested positive for E. coli after eating at Evergreens restaurants in Seattle this month, which public-health officials believe is unrelated to the national outbreak associated with romaine lettuce.

Officials believe the outbreak may be associated with Evergreens, as the six people ate at the salad chain’s restaurants from Nov. 5 to Nov. 11, according to a statement from Public Health — Seattle & King County. Their results are not yet conclusive, though, and one other person who appears to be part of the outbreak did not report eating at the restaurant. Many of the people who became ill also reported eating raw vegetables that weren’t from Evergreens.

Test results from four cases suggest a common source that differs from that in the outbreak associated with romaine lettuce grown in Salinas, California, that has caused two King County resident to become ill.

Officials are awaiting results from the other cases but believe the outbreak could be a result of contaminated product delivered to the chain.

The six people reported eating at the chain’s restaurants in Pioneer Square, the University District, Chinatown International District and downtown (on Third Avenue and Marion Street), all of which have excellent food-safety ratings, according to Public Health. The agency is working with state and federal agricultural officials to see if they can identify the source of the contamination through the supply chain.

E. coli symptoms include bloody diarrhea, stomach cramps and nausea, and they usually last several days, although people can still spread the infection after that. One person was hospitalized from the outbreak, but all have recovered, according to Public Health.

Investigators who visited the Evergreens locations did not observe practices that are associated with the spread of E. coli, according to Public Health. Evergreens restaurants threw out romaine lettuce from their stores in accordance with recommendations from the Centers for Disease Control and Prevention.

To reduce the risk of contracting E. coli, officials say people should avoid consuming undercooked or unpasteurized foods and beverages, regularly wash surfaces that contact food, wash hands before preparing food and wash fresh produce.