Italian prosecutors have opened a murder investigation following the deaths of a mother and her teenage daughter after a pre-Christmas meal last year.

Sara Di Vita, 15, and her mother Antonella Di Ielsi, 50, fell ill after a lunch in their hometown of Pietracatella, a small municipality 260km (161 miles) south-east of Rome.

Sara's father, Gianni, the former mayor of Pietracatella, was also taken to the hospital but later recovered. The couple's eldest daughter was not there that day.

Doctors initially attributed the symptoms to food poisoning, but the new focus comes after laboratory tests revealed the presence of a deadly poison, ricin, Italian media say - even though police still do not have a suspect.

Doctors are said to have thought the infection originated from either fish or mushrooms and discharged the mother and daughter after they went to the hospital.

However, their symptoms quickly worsened, and the pair were soon re-admitted.

Dr Vincenzo Cuzzone, head of the intensive care unit at Cardarelli hospital in nearby Campobasso, told Italian media that liver failure occurred first and was followed by multi-organ failure "at truly unparalleled speed".

Di Vita and Di Ielsi's deaths were initially attributed to medical negligence, and doctors who discharged them were placed under investigation for manslaughter, Italian media report.

They published the test results carried out at laboratories in Italy and Switzerland.

Ricin, a highly toxic chemical, is a poison found naturally in castor beans.

Ingesting a tiny quantity can cause rapid organ failure and lead to death. There is no known antidote for ricin poisoning.