CONCORD, N.H. (AP) — A state investigation has been unable to identify the source of a norovirus outbreak that sickened 20 people and led to one death at a restaurant co-owned by a Democratic congressman.

Eighteen of the 20 people who became ill were attending a private function Nov. 24 at the Puritan Backroom in Manchester and two others worked the event. The restaurant is co-owned by Rep. Chris Pappas and is a frequent stop for presidential candidates of both parties. It has been in his family for more than a century and is famous for its chicken tenders.

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In a report completed last week but released Thursday, the state Bureau of Infectious Disease Control Infectious Disease Surveillance Section concluded that it was unable to find the source of the outbreak. It found no violations at the restaurant during an environmental assessment following the outbreak nor were any workers sick during the event. There also were no reports of other groups who used the venue that day falling ill nor reports of norovirus in the community around that time.

The nature of the event was not released, but it didn’t appear that any current presidential candidates attended.

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The manager of the Puritan Backroom, Erik Zink, said in a written statement after the outbreak that the restaurant’s top priorities are promoting health and excellent food safety and that he was confident it was doing everything possible to maintain those practices.

Zink also said the restaurant had reached out to the family of the person who died to express “our condolences” for the death, which he blamed on unidentified causes.

The symptoms of norovirus include diarrhea, vomiting, nausea and stomach pain, health officials said. The state sees 50 to 75 norovirus outbreaks every year.

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