In-shell hazelnuts taken from the home of a case patient infected with E. coli tested positive for the bacteria, a genetic match to a strain that sickened that person and six others, Minnesota Department of Health officials confirmed Tuesday.

But while the positive sample now proves hazelnuts were responsible for a three-state outbreak of E. coli O157:H7, some companies involved in the distribution chain have apparently resisted efforts to trace the contamination back to the source.

According to a report Tuesday by Capital Press, George Packing Co. of Newberg, Oregon, has refused to give a list of its farmer suppliers to the U.S. Food and Drug Administration, which is trying to determine where and how the nuts became contaminated. The company’s Shaun George said he believes other packers also have declined to reveal their grower-suppliers.  “I think it would be a disastrous thing to do that,” George told Capital Press.

All seven of the individuals sickened by E. coli told public health investigators they’d eaten hazelnuts, and most had purchased the unshelled nuts from bulk bins in retail stores. Investigators tracked those bulk nuts to a Los Angeles-based company, DeFranco and Sons.

The epidemiological link between hazelnuts and the E. coli infections in Michigan, Minnesota and Wisconsin was enough to prompt DeFranco and Sons to voluntarily announce a “precautionary” recall of the in-shell nuts on March 4.

DeFranco, however, was just a “bag in, bag out”  distributor for nuts it had received from George Packing, which in turn got the nuts from others.