PARIS, May 12 (Reuters) - French prosecutors have launched a preliminary criminal investigation after E. coli infections linked to a pizza factory for Nestle's Buitoni brand could have led to the death of one person.

An investigation was opened on Thursday on charges of the involuntary manslaughter of one person, the injuring of 14 others and breaches of food safety requirements, a spokesperson for the Paris prosecutor's office said.

Nestle France was not immediately available for comment.

"The managers of Nestle and Buitoni must be brought to justice and it would be intolerable to sweep this affair under the carpet," said Pierre de Buisson, one of the lawyers of the victims families.

Children who ate the contaminated pizza have suffered brain, heart and lung injuries, he added without specifying details.

Sante Publique France, a health ministry body which investigated the outbreak, said on May 10 that in total, 56 E.coli cases were confirmed, with almost all of the victims aged between one and 17 years.

Escherichia coli, or E.coli bacteria, normally live in the intestines of healthy people and animals. Although many strains of the bacteria are harmless, certain strains can cause severe abdominal cramps, bloody diarrhea and vomiting.

Italy's Buitoni, which produces noodles, ready-made pasta sauces and pizzas and was acquired by Swiss food behemoth Nestle in the 1980s, issued a recall of it's Fraich'Up pizza line after the first E. coli cases were detected in March.

"To date, the origin of the bacteria present in the Fraich'Up pizza are still unknown", Buitoni said in a statement posted on the brand's French website https://www.buitoni.fr/fr/communication-rappel-fraichup.

Former employees of the pizza plant where the cases emerged told French media about catastrophic hygiene conditions.

"You go out with your shoes to smoke and you come back (to the factory floor) with them. People didn't wash their hands," one ex-staffer, who spoke on the condition of anonymity, was cited as saying by France's Liberation newspaper.

France's RMC and BFM radio on Thursday first reported the news of the investigation. (Reporting by Tassilo Hummel; Editing by Franklin Paul, Barbara Lewis and Bernard Orr)