A witness who claimed to have seen Laci Peterson (search) hours after police allege her husband killed her has died, potentially dealing a blow to Scott Peterson's (search) defense.

Vivian Mitchell, 78, died early Wednesday of natural causes, according to her family.

She was one of three people who said she saw Laci Peterson alive midmorning on Dec. 24, 2002. Authorities assert Scott Peterson killed his pregnant wife the night before or early that morning, then drove to San Francisco Bay (search) and dumped her body.

Prosecutors have charged Scott Peterson, 31, with two counts of murder. His trial may start later this month in San Mateo County, where it was moved because of extensive publicity in Modesto, where the couple lived. If convicted, Peterson could face the death penalty.

Peterson's defense already has started questioning potential jurors in San Mateo County.

Paul Strand, a political science professor at San Diego State University, confirmed Thursday he's conducting the poll but refused to elaborate, citing a gag order. Strand also conducted one of two surveys the defense cited in its successful bid to move the trial out of Stanislaus County.

Mitchell, who lived nine blocks from the Petersons' home, was one of three people who said they saw Laci Peterson walking her dog about 10 a.m. Christmas Eve 2002. Scott Peterson says he left to go fishing on San Francisco Bay earlier that morning.

Geragos and John Goold, a Stanislaus County prosecutor, refused to say Wednesday whether Mitchell had been questioned under oath at any point.