The lead detective assigned to investigate Laci Peterson's (search) disappearance backed up a key defense theory Monday that the pregnant schoolteacher had planned to walk the couple's dog the morning she vanished.

Prosecutors claim Laci had stopped walking the dog weeks earlier at her doctor's request, and that Scott Peterson (search) set the dog loose to make it appear someone abducted her.

Defense lawyer Mark Geragos (search) also used Modesto police Detective Craig Grogan (search) to attempt to show that Laci Peterson knew about her husband's relationship with massage therapist Amber Frey. The affair was his motive, prosecutors allege, for the murder.

Grogan, a prosecution witness, recounted last week why authorities maintain Scott Peterson is the only person who could have killed his wife. But Geragos used Grogan on Monday to offer a host of reasonable explanations for some of Peterson's behavior that police deemed "suspicious."

Prosecutors allege Peterson killed Laci around Dec. 24, 2002, in their Modesto home, then dumped her weighted body into San Francisco Bay. Her remains, and that of her fetus, washed up in April 2003, not far from where Peterson launched his boat that Christmas Eve morning for what he claims was a solo fishing trip.

Defense lawyers maintain someone else abducted and killed Laci while she walked the dog in a nearby park. The dog was found by a neighbor running loose in the street the morning Laci vanished, according to previous testimony.

But Grogan acknowledged Monday that Scott Peterson told police on the first night of the investigation that Laci had planned to walk the dog that morning. Sharon Rocha, Laci's mother, told Grogan that Laci walked the dog every morning, the detective testified.

Geragos also noted that Peterson was cooperative, even as police sought search warrants for his home and a warehouse where he stored the boat prosecutors allege he used to dispose of his wife's body.

"He basically said, 'If you had asked me I would have let you search [the warehouse].' ... Is that correct?" Geragos asked.

"Yes, he said something like that," Grogan said.

Geragos then suggested that Laci knew about her husband's affair.

Grogan testified that a relative of Scott Peterson told him that Peterson had told her Laci knew about the affair with Frey and, according to the police report, she was extremely angry and upset.

Grogan acknowledged the woman told him that "Laci insisted that they not tell the parents about it," Geragos quoted from the report.

"She described Scott as being very excited about becoming a father and that he and Laci seemed very happy together?" Geragos asked the detective.

"That's correct," Grogan said.

Geragos then attacked another key point in the prosecution's case against Peterson -- that he kept his newly purchased boat a secret from Laci.

Grogan acknowledged that Peterson told him early in the investigation that Laci knew about the boat and had been at the warehouse where he stored it on Dec. 20, four days before she vanished.

Another detective testified previously that a woman said she saw Laci at the warehouse on that day.

It's a crucial point because the prosecution's only piece of potential physical evidence linking Laci to the boat is a strand of dark hair found in a pair of pliers on the vessel. Experts have testified that DNA testing indicates the hair likely came from Laci.

But defense attorneys have argued that the hair may not be Laci's, that it may have fallen from Scott Peterson's clothing or even came from Laci herself when, they say, she saw the boat for the first time on Dec. 20.

Peterson faces the death penalty or life without parole if convicted.

In later testimony, Geragos suggested Laci Peterson's murder may have been the result of mistaken identity.

He noted that authorities had received a tip early on from a lawyer he identified only as "Michelle" who worked as a prosecutor in Merced County and lived near the Petersons' home in Modesto.

Geragos described her as looking very similar to Laci Peterson and noted the woman had given birth to a child in October 2002. The woman walked her dog -- the same kind the Petersons had -- in the Petersons' neighborhood.

The woman had recently been threatened after prosecuting a case.

She called police after Laci vanished to report that the alleged abductors "might have mistaken Laci Peterson for her, correct?" Geragos asked the detective.

"Correct," Grogan replied.