Updated

A former hitman who admitted killing 20 people says he decided to testify against James "Whitey" Bulger after learning Bulger and his partner were FBI informants.

John Martorano served 12 years in prison after striking a cooperation deal with prosecutors.

Martorano took the witness stand Tuesday against Bulger. He described Bulger and his partner, Stephen "The Rifleman" Flemmi, as his "partners in crime," his best friends and the godfathers of his children.

Martorano says he decided to become a government witness against them because they violated his trust by becoming informants, something he said "sort of broke my heart."

Bulger, the former leader of the Winter Hill Gang, is charged in a 32-count indictment that accuses him of participating in 19 murders in the 1970s and '80s. He is also charged with extorting bookmakers, drug dealers and others running illegal businesses.