SAN FRANCISCO (Reuters) - Nearly one year after a pregnant Laci Peterson disappeared from her California home, prosecutors on Wednesday will finally lay out evidence they say shows Scott Peterson killed his wife and unborn son.

The eagerly-anticipated preliminary hearing in Modesto is expected to last up to five days after which Stanislaus County Superior Court Judge Al Girolami will decide whether there is enough evidence to merit a trial for the 31-year-old salesman.

Scott Peterson, who has been in jail since his April arrest, is charged with two counts of murder and could face the death penalty if convicted in the case that has attracted exhaustive press coverage. He has pleaded not guilty.

A gag order has also kept a lid on much of the evidence so far and lawyers following the case said the preliminary hearing will likely showcase new details.

"There are going to be some surprises," said Jim Hammer, a prosecutor with San Francisco's district attorney's office. "It is a little like we have been waiting for the movie to start and we have been watching previews for the past month."

He pointed out that wiretaps, which the defense has been trying to suppress, would let people hear Scott Peterson's words as police were investigating his wife's disappearance from her home some 85 miles from San Francisco.

There is also rising speculation prosecutors will call the woman with whom Peterson was having an affair -- Amber Frey -- to testfy.

"Although I don't think he confessed to the murder, I think the prosecution has some highly damaging or at least embarrassing evidence in the terms of Scott Peterson's words on those tapes," Hammer said.

A heavily pregnant Laci Peterson disappeared on Christmas Eve from her home in Modesto, sparking an intense search among members of the closely-knit central California farming community and generating national media headlines.

The search ended in April when two decaying corpses washed out of San Francisco Bay near where Scott Peterson had said he had gone fishing the day Laci disappeared.

While legal experts say the fertilizer salesman will almost surely face a trial since the standard of proof in a preliminary hearing is far less than that needed to convict someone, they add Peterson can still gain some benefit from the proceeding.

Peterson attorney Mark Geragos, who has suggested in the past a Satanic cult could be responsible for Laci's death, will be able to preview the evidence and witnesses before a trial, said San Francisco criminal defense attorney Jai Gohel.