FDA Associate Commissioner for Foods David Acheson told reporters in a conference call Friday the chain's identity would remain confidential, the Los Angeles Times reported Saturday.
An FDA spokeswoman said in an e-mail that the confidentiality was due to the fact investigators were more concerned about the tomatoes causing the outbreak rather than the sites serving them.
"The restaurants are not the problem. The tomatoes are what is making people sick across the country, and we don't know where they came from," spokeswoman Julie Anne Zawisza said.
The Times said in addition to nine cases of food poisoning in Chicago, none of which resulted in hospitalization, there have been a total of 228 salmonella cases in 23 U.S. states since mid-April. Acheson referred to nine cases in Friday's conference call but it was not known if they were the same cases as those reported by Chicago health officials.
Acheson said most of the tomatoes produced during the outbreak were likely from either Florida or Mexico, the newspaper reported.