Updated 11/08/12 – 4:37 p.m.

CHICAGO (CBS) — A new search for the remains of Drew Peterson’s missing fourth wife was in its fourth day on Thursday.

WBBM Newsradio’s Mike Krauser reports the FBI and Illinois State Police, aided by a cadaver dog, have been searching Hammel Woods in Shorewood for the body of Stacy Peterson.

They have been seen carrying shovels into the 450-acre property.

LISTEN: WBBM Newsradio’s Mike Krauser Reports

State Police spokeswoman Monique Bond would not confirm any details of the investigation, adding police are “taking advantage of the law enforcement resources for training purposes.”

However, CBS 2′s Dana Kozlov reports sources confirmed the search teams were looking for Stacy Peterson’s body, and won’t pull up stakes for good until every one of the 450 acres there have been searched.

The FBI’s 20-member evidence response team has been conducting the search since Monday. They were focusing on Hammel woods, because that is where Stacy’s last cell phone activity was recorded in 2007.

A source said the new search was conducted because authorities were able to get access to the best cadaver dogs, a pair of highly trained Springer Spaniels.

The source said, if one dog finds something, the team would bring the second dog over to verify the find, and then secure the scene for a forensics team.

A source said agents were concentrating on areas off trails, where someone could have taken, and dumped or buried a body. They are expected to be back again on Friday.

If there is any other reason for the timing of the search, no one was saying.

Stacy Peterson disappeared a little over five years ago. Police believe she was murdered by her husband, Drew Peterson, who has been declared a person of interest in Stacy’s disappearance, but not been charged.

Peterson, a former Bolingbrook police sergeant, was convicted earlier this year in the murder of his third wife, Kathleen Savio.

He has not been charged in Peterson’s disappearance, and he has contended that Stacy ran off with another man.