A Spanish magistrate has opened an investigation after one woman died and 28 other diners fell ill after eating at a Michelin-starred restaurant in Valencia.

A 46-year-old woman died Sunday after eating at the restaurant RiFF the previous evening with her husband and 12-year-old son, both of whom also fell ill. A Valencia court statement Friday said the magistrate is awaiting forensic reports and the results of an autopsy.

Ana Barceló, the head of Valencia's Ministry of Health, told reporters Thursday that investigators had interviewed 75 people who had eaten at the restaurant from Feb. 13 to Feb. 16 and learned that a total of 29 had suffered food poisoning, including the woman who died. The other 28 people suffered symptoms including vomiting and diarrhea.

People walk by the entrance of the Riff restaurant in Valencia, Spain, on Feb. 22, 2019. Jose Jordan / AFP/Getty Images

Food safety officials inspected RiFF on Monday but were not able to find an obvious cause. Samples from the restaurant's tasting menu were sent to the National Institute of Toxicology and Forensic Science for analysis.

Get Breaking News Delivered to Your Inbox

Some reports have suggested that morel mushrooms may have been to blame, but Barceló said it was "irresponsible to establish the origin" before an autopsy and other tests were completed.

Bernd Knoeller, the German owner and head chef of RiFF, temporarily closed the restaurant Tuesday, saying he was cooperating with authorities who are investigating the exact cause of the outbreak.

"Regardless of the reason that may have caused this situation, I want to convey my deep regret for what happened, hoping that soon all these facts can be clarified," he said in a statement. "I have made the decision that the restaurant remains closed until the causes of what happened are established."

RiFF opened in 2001 and was awarded a star by the prestigious Michelin Guide in 2009, according to the restaurant's website.