Officials said 13 of the people who fell ill ate at the restaurant -- Ixtapa -- located at 303 91st Ave. N.E.
After learning of the findings, the owner voluntarily closed the business and is cooperating with officials to resolve food-safety problems, the Snohomish Health District said.
Public health workers will oversee sanitizing the restaurant and disposing of opened food products. Health District spokeswoman Suzanne Pate said the restaurant was being "very cooperative."
Health workers are now trying to identify the specific food that was the source of the illness. Contaminated lettuce was identified as the source of an E. coli outbreak in Washington earlier this year.
"It's like looking for a needle in the haystack," Pate said.
Ixtapa owners said other restaurants in Snohomish County by the same name have not been implicated in the illnesses.
"Food safety and the health of our customers is the No. 1 priority of Ixtapa's owners and employees," a statement issued by the restaurant said.
"All of our employees are certified through state-approved food safety programs, and we set the highest standards for compliance."
The onset of illnesses occurred between Oct. 7 and Oct. 17. Most of the sickened people ate at the restaurant between Oct. 2 and Oct. 3. Two were hospitalized briefly. No new cases were reported as of Tuesday.
Officials said three of the 17 sickened people did not eat at the restaurant, and health workers were investigating any pattern among them. "Sometimes links emerge later," Pate said. "Or there's a possibility they might be three of the usual cases we see a year."
A fourth person who fell ill has not yet been interviewed.
Gary Goldbaum, director of the health district, said Snohomish County health care providers should be on the lookout for patients with E. coli infections, which can cause bloody diarrhea and stomach cramping with little or no fever.
E. coli refers to a large group of bacteria, some of them harmful. Infections occur when people ingest small amounts of fecal matter, often through unpasteurized liquids and contaminated food and water, or by failing to wash hands after going to the bathroom, changing a diaper or touching farm animals.
People usually recover from the illness, but young and old people have a higher risk of serious complications.