MILAN (Reuters) - A MAFIA informant who disappeared nearly a year ago was tortured and her body dissolved in acid by her former partner and other family members angry that she had cooperated with authorities, Italian police said on Monday.

The woman, 35-year old Lea Garofalo, was forced onto a truck carrying 50 litres of acid, interrogated by her abductors and later killed in a field near Milan in the night between Nov 24 and Nov 25, 2009, investigators told reporters.

Four people were arrested and another two, including Garofalo's former partner, Carlo Cosco, were served arrest warrants while already in jail.

Investigators said Cosco, who fathered Garofalo's daughter, was behind the murder and that this was meant as revenge for the woman having provided information to anti-mafia prosecutors between 2002 and 2009.

Both the Garofalo and Cosco families are linked to the'Ndrangheta organised crime group, which is based in the southern region of Calabria and is now considered the richest and most powerful criminal syndicate in Italy.

Emboldened by a string of high-profile mafia arrests, Prime Minister Silvio Berlusconi often boasts that his government has been the toughest and most successful in fighting mobsters.