State and federal officials won’t say where it came from, but they are blaming whole head green cabbage for an E. coli outbreak this summer that spurred Applebee’s restaurants to temporarily pull certain salads from the menu.

Findings as of the end of August suggest the restaurants were not to blame for the outbreak, which sickened 15 in Minnesota.

“The cabbage was likely contaminated at some point prior to distribution to restaurants,” according to a release from the Minnesota Department of Health.

The restaurants changed suppliers, according to an Applebee’s spokeswoman, and eventually restored all items to their menus.

An investigation into the cabbage supplier is underway by the U.S. Food and Drug Administration, according to Minnesota officials. An FDA spokesman said the agency’s policies prohibit release of details of open investigations.

“The investigation is ongoing, but we anticipate it is nearing its end,” the FDA spokesman said Sept. 2, adding that any updates would be posted on FDA’s Coordinated Outbreak Response and Evaluation (CORE) Web page at tinyurl.com/COREoutbreaks.

The 15 cases of the unusual E. coli 0111 strain that were confirmed between June 25 and July 3 resulted in four hospitalizations, according to the Minnesota Department of Health. Single cases of the identical strain have been confirmed in three other states, but those states have not released information to the public.

“This genetic strain of E. coli 0111 had not been seen in the U.S. previously,” according to the release.

Minnesota officials interviewed 14 of the 15 sick people in their state and found that 13 of them had eaten at nine different Applebee’s restaurants. One ate at a Yard House restaurant. Several of them reported eating the Applebee’s oriental chicken salad.

“The common food item across all foods consumed by cases was green whole head cabbage,” according to the Minnesota Health Department release.

“Minnesota officials traced the cabbage to a common supplier outside of Minnesota and continue to work with the U.S. Food and Drug Administration to investigate its source. The FDA examination of the potentially involved farms is still ongoing.”