(NEXSTAR) – While the cyclospora outbreak linked to lettuce has caused thousands of cases of cyclosporiasis — and likely even more we may never know about — another outbreak with an unknown cause has increased, according to federal authorities.

The U.S. Food and Drug Administration last week added this separate outbreak to its ongoing foodborne illness investigations.

The cyclosporiasis cases in this outbreak have not been linked to the lettuce outbreak, and the FDA says it is still conducting traceback efforts to identify the source. Meanwhile, an additional 21 cases have been linked to the outbreak, bringing the case count to 93.

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There are still six active cyclospora outbreaks under investigation. Three — sickening 18, eight, and two people, respectively — are listed as ended. Only the lettuce-linked outbreak has an identified source.

While U.S. officials have said that domestically acquired cases of the "explosive" diarrhea-causing illness are linked to produce, the source of an outbreak isn't always identified. Because of how long it can take for people to experience symptoms, and the difficulties of sequencing cyclospora, investigations can "take a significant amount of time," Gwen Biggerstaff, the deputy director of the CDC's Division of Foodborne, Waterborne, and Environmental Diseases, said earlier this month.

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There are currently nine states linked to the outbreak caused by iceberg lettuce sourced from Mexico. As of Wednesday, more than 10,000 cases have been reported in Michigan alone.

Meanwhile, health officials have insisted that the "U.S. food supply is safe."

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