An expert on tides and currents testified Monday that the bodies of Laci Peterson (search) and her fetus may have been dumped into San Francisco Bay near the spot where her husband Scott Peterson claims to have gone fishing the day of her disappearance.

Ralph Cheng (search), a hydrologist with the U.S. Geological Survey, said that based on wind and tidal information, the fetus' body — whether still inside the mother or not — was put in the bay between Brooks Island and the Berkeley Marina.

Cheng cautioned that that was the "highest probability" location.

"Can you predict with any certainty within inches or feet where these bodies would have started from?" asked prosecutor Dave Harris.

"No, I'm afraid not," Cheng replied.

He also acknowledged that he could not reproduce the trajectory for Laci Peterson's body, only a possible path for the fetus. Cheng said the fetus' path was easier to determine because it was lighter and unencumbered.

He did not elaborate. But prosecutors believe that the fetus was expelled after the mother's death while her body lay weighted to the bay floor with homemade concrete anchors and that the woman's corpse may have stayed in place longer than that of the fetus.

Defense attorneys attacked the expert's findings as conjecture.

Prosecutors allege the 31-year-old former fertilizer salesman killed his wife in their Modesto home on or around Dec. 24, 2002, then dumped her weighted body into the bay. Her badly decomposed remains — and those of her fetus — washed up in April 2003, not far from the marina where Peterson launched his boat that Christmas Eve morning for what he said was a solo fishing trip.

Peterson told authorities he was fishing around the Brooks Island area in the bay.

Defense lawyers maintain someone else abducted and killed Laci, then framed their client after learning of his widely publicized alibi.

Prosecutors claim an April storm churned up the water, dislodging Laci's body from the bay floor. Cheng said his scientific data supports such a theory.

On cross-examination, defense attorney Mark Geragos (search) attacked Cheng's findings as "assumptions."

"Most scientific information is based on assumptions," Cheng said.

Geragos also got Cheng to acknowledge that this was the first study he had ever done involving San Francisco Bay and objects as big as bodies.

Geragos noted that one of Cheng's own theories for how the bodies could have washed up apart from each other is that they were placed into the bay separately, supporting the defense theory that the fetus was born alive after Laci disappeared. Such a theory, they say, proves Peterson could not be the killer, given Laci's Feb. 10 expected due date.

Later, Michael Griffin took the stand to testify about how he sold Peterson a used Mercedes in April 2003, shortly before Peterson's arrest.

He said Peterson purchased the car under his mother's name — Jacqueline Peterson.

"When I saw the name Jacqueline, I said, 'Are you buying this for your wife?' And he said, 'No, that's my name,"' Griffin testified. "I said, 'Are you French?' He said, 'No, it's kind of a 'Boy Named Sue' type of thing ... I go by Jack."'

Peterson was arrested April 18 in the San Diego area in the Mercedes. Defense lawyers have suggested Peterson was attempting to elude media scrutiny, but prosecutors claim he was trying to dodge police and might have been preparing to flee.