However, it will not be possible to confirm whether she died from poisoning or asphyxiation from particles of vomit in her lungs until an autopsy is carried out.
"We will have to wait...before we can determine whether it was the ingestion of a food that directly caused her death, or whether it prompted a state that led to this fatal outcome," said regional health chief Ana Barceló on Wednesday.
Samples of ingredients have been sent to the National Toxicology Institute, she added.
Ms Fernandez Calvo and her family had opted for the special RiFF taster menu of various Mediterranean-inspired dishes, including one with the perilous morchella mushrooms.
The fungi, also known as true morels, are a delicacy in France but cannot be eaten raw as they contains the powerful hydrazine toxin.
Chefs usually dry the mushroom then rehydrate it with water or milk before a thorough cooking.
Health inspectors are now probing whether the mushrooms served at RiFF on Saturday were prepared correctly or if 'false morels' - an inedible, even more poisonous mushroom - were served by mistake.
Chef Bernd Knöller, who started his culinary career in Britain in the 1980s, expressed his "deep sorrow" for the death of Ms Fernández Calvo.
In a statement released on Tuesday, Mr Knöller said he had taken the decision to close the restaurant until the causes of the food poisoning had been established.
"I have offered my complete cooperation to the Valencian health authority from the very start in order to clear up the facts, with the hope that we can establish the causes as soon as possible," the German chef said.
He added that the restaurant, which earned its first Michelin star in 2009, had been given a clean bill of health in an initial inspection that took place on Monday.
The RiFF menu the family of the deceased chose usually comprises seven dishes, including boletus, pig's ear and rice dishes with seasonal fungi at a cost of €85 per head. A range of selected wines are an option at a further €45.
According to information on the RiFF restaurant website, Mr Knöller travelled to London as an 18-year-old to work at the Kensington Hilton hotel before moving to a position at the Chester Grosvenor.
Later he worked as a chef in Germany, France and Spain, where he opened his first restaurant, El Ángel Azul, in Valencia in 1993. He opened RiFF in 2001.
The Michelin guide praises RiFF for "innovative cuisine...based around the highest quality, seasonal, local product."
Ms Fernandez Calvo was originally from a village called Benavides de Órbigo in the province of León, but had lived for 30 years in Valencia.
