LEWISTOWN, Ill. -- An anthropologist who helped authorities search a central Illinois location for the remains of Drew Peterson's fourth wife says he found only opossum bones.

But anthropologist Alan Harn believes authorities have a "really good" tip about Stacy Peterson, who disappeared in 2007. He's advised investigators to return to the area near Peoria with a device that gets images of what's underground.

Harn told The Associated Press on Monday that he excavated three sites on Saturday identified by cadaver-sniffing dogs.

Drew Peterson is a suspect in Stacy Peterson's disappearance. He's in custody at the Will County Jail while awaiting trial for the murder of a previous wife. He's denied wrongdoing.

Harn says the tip about the site came from "a representative" of someone in jail. He declined to say more.