Lea Garofalo, who was kidnapped, tortured and murdered in 2009 by her former partner for informing on the powerful 'Ndrangheta syndicate, had provided investigators with key details on the murder of seven people between 1989 and 2007, police said.
Based on her testimony, police launched raids across Italy, from Calabria to Emilia Romagna and Lombardia, arresting 17 people accused of murder, harbouring illegal firearms and drug trafficking.
Police did not say why it took some four years to act on her tip-offs to make the arrests.
Garofalo, 35, gave investigators details of a bloody civil war between two 'Ndrangheta families that left 40 dead, and was living under a witness protection programme when she was lured to Milan by her ex-partner who used their daughter as bait.
The burnt remains of her body, initially thought to have been dissolved in acid, were discovered last year.
The 'Ndrangheta - whose name comes from the Greek for courage or loyalty - has a tight clan structure which has made it famously difficult to penetrate.