A "special roll" served at a sushi restaurant inn the US state of Montana is being blamed for killing two people and poisoning at least 30, Insider reported.

Dave's Sushi temporarily closed in April after several customers that dined there fell ill. The restaurant said in an Instagram post that they believe the common ingredient was cultured morel mushrooms, imported from China via a distributor in California.

Donna Ventura, 64, and William Lewis, 74, died days after eating at the restaurant. At least 30 other diners are believed to have been infected, local news station KBZK reported.

Officials said in a statement that preliminary investigative findings point to food containing morel mushrooms as the possible exposure of concern but that no specific pathogen had yet been identified.

Dave's Sushi told the outlet that it was fully cooperating with the investigation and hired a sanitarian consultant to inspect the restaurant.