NEW YORK (AP) -- Seven funeral home directors linked to a scheme to plunder corpses and sell body parts for transplants have secretly pleaded guilty to undisclosed charges, prosecutors announced on Wednesday. The unidentified directors have agreed to cooperate with investigators, prosecutors added. The probe involves a plot to harvest bone and tissue and sell it to biomedical supply companies, Brooklyn District Attorney Charles Hynes said. "It is clear that many more funeral home directors were involved in this enterprise," Hynes said at a news conference. The seven entered their pleas in closed courtrooms and their names were withheld, but defense attorneys said that among those cooperating was the director of a funeral home that took parts from the body of "Masterpiece Theatre" host Alistair Cooke, who died in 2004. The four original defendants in the case pleaded not guilty on Wednesday to enterprise corruption, body stealing and other charges in the new indictment. If convicted, they face up to 25 years in prison. All remain free on bail. Prosecutors allege Michael Mastromarino, a former oral surgeon, and three other men secretly removed skin, bone and other parts from up to 1,000 bodies from funeral homes, without the permission of families. Mastromarino allegedly made millions of dollars by selling the stolen tissue to biomedical companies that supply material for common procedures including dental implants and hip replacements. They were charged in February with counts including body stealing, unlawful dissection and forgery in a case a district attorney called "something out of a cheap horror movie." All the defendants pleaded not guilty before being released on bail. Mastromarino, owner of Biomedical Tissue Services of Fort Lee, New Jersey, allegedly made millions of dollars by selling the stolen tissue to biomedical companies that supply material for procedures including dental implants and hip replacements, prosecutors said. At the time, prosecutors said they had unearthed evidence that death certificates and other paperwork were falsified. In Cooke's case, his age was recorded as 85 rather than 95 and the cause of death was listed as heart attack instead of lung cancer that had spread to his bones. Other evidence includes X-rays and photos of exhumed cadavers show that where leg bones should have been, someone had inserted white plastic pipes -- the kind used for home plumbing projects, available at any hardware store. The pipes were crudely reconnected to hip and ankle bones with screws before the legs were sewn back up. Copyright 2006 The Associated Press.