As Drew Peterson faces murder trial in death of third wife, a question lingers: Where is Stacy, wife No. 4?

Opening arguments under way in Drew Peterson murder trial

The former suburban Chicago police officer is charged with first-degree murder for the 2004 death of his third wife, Kathleen Savio. He was charged after his fourth wife, Stacy Peterson, vanished without a trace in 2007.

While an Illinois court prepares for the murder case against former police officer Drew Peterson, who is accused of killing his third wife, police are probing his possible role in the disappearance of wife No. 4.

Peterson, 58, is charged with the 2004 death of his third wife, Kathleen Savio, which was deemed accidental until police began investigating the 2007 disappearance of his fourth wife, Stacy Peterson. With the former suburban Chicago police officer a defendant in one murder case and a suspect in another, Stacy Peterson's family is convinced of his guilt and confident they will see him tried for her murder.

"I think in time the case will be resolved and hopefully he'll go to trial for that too," said Pam Bosco, a spokeswoman for Stacy Peterson's family. "They have a dedicated force and are not going to give up until they find out what happened to Stacy," she said of investigators.

"He has not been charged because we don’t have a body," added Stacy Peterson's aunt, Candace Aikin, "But I believe that things are progressing with the investigation in a good way."

During the first day of jury selection Monday, a judge and attorneys removed pool members who saw a TV movie about the case or came to court already convinced he's guilty.

Legal experts say the court faces an uphill battle in selecting jurors who have not been subjected to intense media coverage on the case and who have no pre-conceived notions of Peterson's guilt or innocence.

"In a high-profile case whose circumstantial allegations are tailor-made for TV -- a police officer suspected of murdering multiple wives -- jury selection is extremely difficult," said Mark Bederow, a New York-based criminal defense attorney and former prosecutor.

"It is virtually impossible to find a person in that community who hasn't heard of the case and formed some opinion about it," Bederow told FoxNews.com. "The difficult task for the defense is finding jurors who it truly feels believe in the system -- and especially in a circumstantial case -- will hold the prosecution to its heavy burden of proof."

A full jury was selected Tuesday, including a part-time poet and a man in his 70s who takes flying lessons and who played baseball on his college team.The last members of the panel were selected Tuesday afternoon. Seven men and five women are on the jury. Nine jurors are white, three are African-Americans and one is Hispanic.

Attorneys have until next Tuesday to prepare their opening statements, and prosecutors are expected to call their first witnesses later that same day.

In one-by-one questioning, defense attorneys asked prospective jurors if they watched the 2011 cable TV movie starring Rob Lowe as Peterson. They also asked them whether they have been through acrimonious divorces.

Kathleen Savio's body was found in a dry bathtub in her home, her hair soaked in blood, just before her divorce settlement with Peterson was to be finalized. He allegedly feared the settlement with the 40-year-old Savio would wipe him out financially.

Stacy Peterson, Drew Peterson's fourth wife, disappeared on Oct. 28, 2007. Peterson claims that the 23-year-old mother of two ran off with another man. She has never been found, and investigators have said they believe she is dead.

Bosco told FoxNews.com that Peterson "would never have left her children."

"She was an amazing mom," Bosco said, adding that Peterson "made many statements expressing her fears about Drew."

More than 200 prospective jurors were summoned to hear the long-delayed Peterson case in 2009 and were instructed at the time to meticulously avoid all media coverage about it.

Most of the 40 potential jurors questioned Monday said they had heard at least snippets of news about Peterson over the three years, but the majority insisted that whatever they heard wouldn't preclude them from giving him a fair trial.

Asked if she was able to avoid seeing intense media coverage of the case, one woman later struck from the pool said, "It's right there in front of you -- it's hard not to (see it)."

Three people who said they watched the entire TV movie -- in apparent violation of court instructions -- also were dismissed. One was a professional plumber who said he went out of his way to watch the movie.

"He looked guilty," the man said, referring to the conclusion he drew from the movie. In his questionnaire, he also wrote that he spoke to his girlfriend about the movie later.

"We discussed how he done it," he wrote, referring to how they thought Peterson had committed the killings.

Peterson, who shaved his trademark mustache for the trial, has appeared fully engaged in the jury-selection process -- studying potential jurors as they answer questions and making suggestions to his attorneys about questions to ask.

Bederow said Peterson's involvement in the jury-selection process is typical in a high-profile murder case.

"It is quite common for a defendant like Peterson to be actively engaged in jury selection," Bederow said. "He was a police officer for years and has a distinct advantage of knowing the communities from which the jurors will be selected. Additionally, engaging in ordinary social behavior, such as conferencing with his attorney in the presence of jurors, humanizes him, which is especially critical for a defendant accused of violent murders."

Aikin, meanwhile, said the Stacy's Peterson's unsolved disappearance five years ago has had a devastating impact on the family.

"Stacy was vivacious and full of energy," Aikin said. "She was happy and bubbly and loved to be around people.

"She loved her family. They were No. 1 to her," she said.

FoxNews.com's Cristina Corbin and The Associated Press contributed to this report.