Defense attorneys for a former Illinois cop on trial for killing his third wife claimed Tuesday their client was framed for an "accident," while prosecutors argued it was a cold-blooded murder.

Drew Peterson, a 58-year-old former police officer, is on trial for first-degree murder in the 2004 death of his third wife, Kathleen Savio, whose body was found slumped forward in a dry bathtub, her hair soaked in blood. Savio's drowning death was ruled accidental until police began investigating the 2007 disappearance of his fourth wife, Stacy Peterson.

Mary Pontarelli, Savio's neighbor and friend who found her body, was the first witness to take the stand. Pontarelli had gone into the house looking for Savio, and after seeing her in the tub, said that she "ran out of the bathroom then and threw myself on the ground and started screaming."

Pontarelli described what she saw: a cut in her head, dried blood on her hair, and bruising on her wrists. And when asked to identify Savio she started crying.

Prosecutors said during opening arguments Tuesday that Savio's death was a murder staged to look like an accident. They suggested Peterson killed Savio because he feared their pending divorce settlement would wipe him out financially.

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Defense attorneys, meanwhile, told jurors that there is no physical evidence linking Peterson to Savio's death.

"There's no murder weapon, no crime scene, no forced entry, no broken glass, no sign of a struggle, no blood splattered, no broken glass or items," defense lawyer Joel Brodsky told the courtroom. "The bathroom was in perfect order."

While Peterson is on trial for the murder of Savio, police are still actively probing his possible role in the disappearance of Stacy Peterson.

Peterson claims Stacy, a 23-year-old mother of two, ran off with another man, but authorities have named him a suspect in her disappearance. They believe Stacy Peterson is dead, but her remains have never been found.

The real-life drama inspired a TV movie and a national spotlight was put on the case, with speculation about whether Peterson used his law-enforcement expertise in a bid to get away with the murder of Savio, 40, and to make Stacy Peterson disappear.

Tuesday's opening statements in Joliet, Ill., pit the dry but dogged James Glasgow, Will County's state's attorney, against flamboyant defense lawyer Joel Brodsky, both of whom have staked at least part of their reputations on the final result.

Peterson's attorney is seeking to tell jurors, who include a part-time poet, a letter carrier and a research technician whose favorite TV show is "Criminal Minds," the life story of his client and Savio.

The defense has described Savio's death as a tragic accident, and they have said Stacy Peterson, whose body has never been found, may have run off with another man.

Glasgow may face the greater challenge, legal experts say.

A botched initial investigation into Savio's death left prosecutors with scant to no physical evidence, forcing them to rely heavily on hearsay evidence -- statements not heard directly by a witness -- which is normally barred at trials.

But New York-based criminal defense attorney Mark Bederow told FoxNews.com that, "Circumstantial evidence can be enough to convict in certain situations."

"Certainly a pending divorce settlement which might ruin Peterson financially provides a strong circumstantial motive for a homicide," Bederow said. "Moreover, if credible medical evidence establishes that the death in the bathtub was a homicide, then the whole theme of the defense — “it was an accident” — crumbles and the defense loses all credibility with the jury, which would greatly harm Peterson.

"The task for the prosecution will be credibly explaining why the homicide was mischaracterized as an accident," he said.

Glasgow has said previously that Savio and Stacy Peterson will effectively speak to jurors through witnesses who can describe how Drew Peterson allegedly told his wives he could murder them and make it look like an accident.

But Judge Edward Burmila has said he would decide what hearsay statements to admit only as testimony proceeds, so Glasgow will have to decide whether to risk mentioning statements to jurors that the judge might later prohibit.

Brodsky told jurors during his opening remarks that they will hear only "myth, rumor, innuendo and hearsay" during the trial.

In the case of Stacy Peterson, Burmila has warned prosecutors they can't tell jurors Drew Peterson is responsible for her disappearance or refer to authorities' belief that she is dead.

Prosecutors have said they believe Peterson killed Stacy, in part, because she knew about Savio's death.

Stacy's family, meanwhile, says they are convinced of his guilt and confident they will see him tried for her murder.

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

Fox News' Marla Cichowski, Cristina Corbin and the Associated Press contributed to this report.