Letter Places Stacy Peterson in Ill. Grocery Store

Investigators plan to review surveillance video from a supermarket where missing mother of two Stacy Peterson reportedly was spotted more than a month after she vanished from her suburban Chicago home.

This latest twist on the investigation came after Drew Peterson, Stacy's husband, received an anonymous letter at his home that said Stacy, his fourth wife, had been seen inside a Peoria, Ill., grocery store.

"It is very specific as to time, place, even in the grocery store what aisle she was standing in, what clothes she was wearing," said Drew's attorney Joel Brodsky.

Also, the FBI has now joined the Illinois State Police in the search for Stacy. The FBI will add additional manpower to the case, which has grabbed the media's attention during the last several weeks.

"We really have an international reach, and those resources can prove invaluable at times," said FBI spokeswoman Ross Rice.

Drew, 53, has been the only suspect named publicly in the 23-year-old's late October disappearance, though he claims his wife ran off with another man and isn't missing at all.

The former police sergeant, who resigned after public scrutiny of the case, said he believes his wife left the same way her mother did when she was young.

"She left on her own accord, so I don't know [where she is]," Drew told ABC News before learning about the news of the possible grocery store sighting.

But Stacy's family said she would never inflict the same type of pain on her small children as she felt as a 16-year-old when her mother left.

"There's no way she'd ever walk away from those kids — no way," said Stacy's stepsister Kerry Simmons. "She would call at least somebody to say, 'Don't worry. I'm OK.'"

Friends and family have said Stacy told them that the couple's marriage was abusive and that she would seek a divorce.

Diane Wetendorf, who runs a support group for victims whose abusers are police officers, said even when spouses run away, they are often tracked down quickly.

"One of the myths that's out here is that women can just disappear into this kind of underground, and the reality is that there is no underground, especially if a police officer has access to all kinds of information," Wetendorf said.

Drew's first wife, Carol Brown, told "Good Morning America" earlier this month her ex-husband could be overbearing, but she added she would be surprised if he was involved in Stacy's disappearance or in the suspicious death of his third wife, Kathleen Savio.

After Stacy's disappearance, investigators reopened the Savio case. Her body was exhumed when police reopened the investigation, which originally was ruled an accidental death by drowning. After examining her body, Michael Baden, a renowned forensic pathologist, said that Savio's death was a homicide.