Updated

Drew Peterson's son testified Wednesday at his father's murder trial that he never believed his dad killed his mother.

The 19-year-old child of the former suburban Chicago police officer and his third wife, Kathleen Savio, was the last witness to testify for the defense, which rested its case Wednesday after presenting three days of testimony. Just before the defense, Drew Peterson stood in the courtroom to tell the judge he had chosen not to testify.

Peterson has pleaded not guilty to first-degree murder in Savio's 2004 death, which was initially ruled an accident. Her death was re-examined and reclassified a homicide only after his fourth wife, Stacy Peterson, disappeared in 2007.

Thomas Peterson, now a student at the University of Pennsylvania, was 11 when his mother was found dead in her bathtub. At the time, he and his brother were staying with Drew and Stacy Peterson at their home just blocks from Savio's house.

"I believe that my dad is innocent," he said firmly when a defense attorney asked why he was in court.

"Are you here to support your father?" defense attorney Joel Brodsky moments later.

"Yes, sir," he said.

Thomas Peterson told jurors he saw no change in his father's demeanor around the day his mother died, saying Drew Peterson was his usual jovial self.

"There was nothing out of the ordinary," Thomas Peterson said. "I would remember if there was."

He said that when his father broke the news about their mother's death to them, he seemed genuinely distraught.

"I have never seen someone so shaken," Thomas Peterson told jurors. "It was troubling to see."

Earlier Wednesday, an attorney told jurors that -- shortly before she vanished in 2007 -- Stacy Peterson asked about whether she could squeeze more money out of her husband in divorce proceedings if she threatened to tell police he killed Savio.

Defense attorneys called the divorce attorney to the stand in a bid to dent the credibility of testimony from others who have testified that Stacy Peterson implicated her husband in Savio's death.

Authorities presume Stacy Peterson is dead, although her body has never been found. Drew Peterson is a suspect in her disappearance, but he has never been charged in that case.

Divorce attorney Harry Smith testified that Stacy Peterson telephoned him in the fall of 2007 to discuss a divorce. When she did, she brought up Savio's death and wondered aloud if she could use it as leverage as she launched the legal proceedings, Smith said.

"She wanted to know if, in my opinion ... the fact that he killed Kathy (Savio) could be used against him," Smith testified. Later he added, "She said, `If we threatened to do this, could we get more money."'

Calling Smith to the stand was a risky move for the defense. While his testimony suggested Stacy Peterson was mulling a possible shakedown attempt, it also emphasized that she seemed convinced her husband did kill Savio. And it risked raising the question in jurors' minds about just where Stacy Peterson is.

The judge has barred prosecutors from mention Stacy Peterson's disappearance during the trial. But Smith came close to broaching that issue when he testified Stacy Peterson never got around to hiring him. When defense attorney Joel Brodsky asked why, Smith stammered uncomfortably, apparently understanding the answer would be inadmissible. After a hastily called break, Brodsky withdrew the question.

Another time, Smith said he told Stacy Peterson during the phone conversation that she should "be careful" -- an apparent reference to how Drew Peterson's could do her harm. When Brodsky said Smith had previously testified that he told her to be careful because she could be arrested for attempted extortion, the witness denied it.

"I did tell her to be careful, but it wasn't about extortion," he said. He didn't elaborate.