Some 13 million pounds of the nuts supplied by Paramount Farms of California and packaged under a variety of brands have been recalled. Investigators have found no trace of salmonella in any of the recalled almonds or at Paramount, investigators say, and it’s possible the outbreak’s source may never be found.

Authorities said all of the Washington cases stemmed from eating almonds sold at Costco under the Kirkland Signature brand. Costco has mailed about 1.2 million letters to members worldwide about the recall, reports the Seattle Times.