SNOWTOWN, Australia, May 24 (UPI) -- At least six bodies have been found in acid-filled vats in an abandoned bank and two others have been dug up from a backyard grave as the investigation into Australia's worst serial killing continues. Police say today that social security fraud may be the motive for the killings. Three men from Adelaide's northern suburbs have been charged with murder in the investigation. Last week police found six plastic vats containing acid in the vault of an abandoned bank in Snowtown. According to the Sydney Morning Herald, police have recovered 15 human feet from the vats, suggesting at least eight victims, although the official death count at the bank is six. The vats had been in the empty building for up to three months. Over the weekend, investigators dug in a backyard of a home in which one of the suspects used to live in and found plastic bags containing human remains. Police believe there were two people buried in the yard in suburban Adelaide. Detective Superintendent Paul Schramm, the officer in charge of the investigation, tells The Australian newspaper: ``Social security fraud is certainly part of our investigation. It started as a missing persons investigation and gradually built as link were established between various people.'' Among the missing persons thought to be a victim in the Snowtown killings is Elizabeth Haydon, a 37-year-old mother of eight who disappeared last year. She was married to one of the suspects. John Justin Bunning, 32; Mark Ray Haydon, 40; and Robert Joe Wagner, 28; were charged on Friday with killing an unknown person between Aug. 1, 1993 and last Thursday. The men, all from Adelaide's northern suburbs, were remanded into custody until July 2. The worst serial murders in Australia are the case in which seven backpackers were killed from 1989-92, and the so-called ``granny killer'' slayings in which six elderly women died from 1989-90.  