Illinois State Police told FOX News on Sunday that "new information" prompted its appeal for more information about possible sightings of the cars that belong to Stacy Peterson or her husband, Drew, around the time that the woman vanished.
Investigators are asking anyone who may have seen the vehicles on Oct. 28 or 29 — the time the 23-year-old Bolingbrook mother's disappearance — to come forward.
Police have released photos of the vehicles: A dark Blue 2005 GMC Yukon Denali belonging to Drew and the second a purple 2002 Pontiac Grand Am, belonging to Stacy, and asked the public to call its tipline at (815) 740-0678 or "America's Most Wanted" at (800) CRIMETV.
Both vehicles were impounded by police on Nov. 1., and are still in police possession because of search warrant seizures.
The appeal came as emergency workers and volunteers searched woods, lakes and ponds this weekend.
The FBI is scheduled to join in the search for Peterson on Monday at the request of state police. Federal investigators are expected to add more searchers and technical assistance during water searches.
Drew Peterson's attorney, Joel Brodsky, welcomed the FBI's involvement in the case, telling FOX that once the FBI starts to question people, he's hopeful this case can start from scratch and refocus on Stacy Peterson as a missing person's case.
"I'm glad the Bureau is coming in. They're (the FBI) is not out to railroad anyone. They're there to get to the bottom of this story."
State Police are already working with investigators from around the state, and have classified the case as a top priority.
Meanwhile, FOX News’ Laura Ingle spoke briefly with Drew Peterson off-camera Sunday in front of his home. When asked about reports that he was seen with another man hauling a large blue barrel to his sport-utility vehicle the day his wife disappeared, Peterson said he has nothing to say about that.
In addition to the missing blue barrel, police are reported to be looking for scuba diving weights missing from Drew's house, which may have led investigators to search numerous ponds. When asked about his missing gear, Peterson told FOX his scuba weights were not missing.
Stacy was last seen by family members on Saturday evening, Oct. 27. Drew Peterson, a former Bolingbrook police officer, has been named a suspect. His third wife, Kathleen Savio, was found dead in her bathtub in March 2004. Savio's death was ruled accidental at the time, but a recent autopsy showed evidence of homicide. Drew Peterson has denied involvement in either case.
Laura Ingle and Jeff Goldblatt contributed to this story.