Testimony in Scott Peterson's (search) double-murder trial turned Tuesday to the discovery of the decomposed bodies of his wife and the couple's fetus along San Francisco Bay.

Some jurors appeared distressed as lawyers displayed pictures of the remains, either rubbing their foreheads or covering their mouths.

Peterson did not look at the large screen where pictures of his wife and their son were displayed. Laci Peterson's (search) parents, who have been present throughout the trial, left the courtroom.

Michael Looby said he and his wife were walking their dog along the bay April 13, 2003, when he saw the remains in a marshy area. "It was a body of a small baby," he testified.

Last week, prosecutors focused on witnesses who testified about Peterson's affair with a massage therapist.

Prosecutors allege the affair with Amber Frey (search) was Peterson's motive for the slaying of his pregnant wife on or around Dec. 24, 2002. They say he then dumped Laci Peterson's body in San Francisco Bay (search). They theorize the male fetus was expelled after her death.

Defense lawyers say someone else abducted Laci Peterson near the couple's Modesto home as she walked her dog, held her captive before killing her and dumped her body to frame Peterson.

In his testimony, Looby said he and his wife asked nearby residents to call 911. A short time later, a fire crew arrived.

The body of Laci Peterson washed ashore a day later not far from where the fetus was found.

Elena Gonzalez testified she found the torso half-submerged in the water along a rocky embankment, and that she saw a dog sniffing around the remains.

Defense attorney Mark Geragos appeared to push his theory that the fetus was killed after birth during questioning of police officer Tod Opdyke, one of the first to respond to the scene.

"Did you notice ... what appeared to be some tape or twine around the baby's neck?" Geragos asked. Opdyke called it a "tapelike substance."

Though Geragos has previously said the fetus had something physically tied around its neck, prosecutors say it was simply debris from the water.

Geragos also questioned police officer Timothy Gard if he found anything in the area resembling the tape or twine wrapped around the fetus' neck. Gard said he did not.

Earlier in the day, California Department of Justice polygraph expert Douglas Mansfield testified that he interviewed Peterson on Dec. 25, 2002, a day after Laci vanished.

Mansfield said Peterson told him neither he nor his wife were having an affair.

Peterson said his pregnant wife had planned to walk the dog in a nearby park the morning she vanished — and that their marriage was fine, said Mansfield. He described Peterson as "very cooperative" during the interview, which lasted nearly three hours.

Mansfield said Peterson spoke of problems the couple had with transients in a nearby park, and that he'd suggested his wife start carrying pepper spray when she walked the dog.

Mansfield was not identified to the jury as a polygraph examiner; he was described only as an employee of the state Justice Department. The context of his questioning of Peterson also was not made clear in court. Polygraph examinations are generally not admissible as evidence.