Illinois State Police plan to investigate an anonymous letter claiming a sighting of missing Stacy Peterson in Kentucky if the original copy is turned over to investigators, FOX News learned Tuesday.

The writer of the letter claims confronting a woman matching Peterson's description in a mall parking lot on Nov. 18. Peterson was reported missing Oct. 29 after she failed to show up at a friend's house.

Sgt. Luis Gutierrez, a spokesman for Illinois State Police, told FOXNews.com that if presented with the letter, investigators would treat it like any other lead. The investigation includes nearly 900 leads, Gutierrez added.

Former Bolingbrook Police Sgt. Drew Peterson, a suspect in his wife's disappearance, denies any involvement and claims she left him for another man. Peterson has not been charged.

Peterson's attorney says Peterson opened the anonymous letter on Sunday. The author claims seeing the woman with another man in the parking lot of a shopping mall.

The writer of the letter claims to have spoken to Stacy. The letter says Stacy told the writer to "just mind your own business" and "please don't ruin my life."

The writer also claims to have photographed Stacy before she got away.