As a murder trial gets under way Monday in the disappearance of Nina Reiser, a 31-year-old mother of two missing since September, defense lawyers are trying to sow doubt about whether she is dead at all.

Reiser's body has never been found — a fact that lawyers for her estranged husband, Hans Reiser, 43, are trying to highlight. The defense attorneys have suggested that Nina Reiser may be alive in her native Russia, where she lived until 1999, and that purported family ties to a Russian spy agency and organized crime may be connected to her disappearance.

Click here to visit NinaReiser.com, which includes photos of her.

The absence of a victim places an added burden on prosecutors, who must rely on circumstantial evidence to persuade jurors not just that her husband committed a homicide, but that a murder was even committed, defense experts say.

"Whenever there isn't a body, there's just lots of arguments that are wide open," said Robert Talbot, a criminal law professor at the University of San Francisco.

Prosecutors did not charge Scott Peterson, now on California's death row for the murder of his wife, Laci Peterson, Talbot noted, until her remains were found on a beach along San Francisco Bay and identified.

The Reisers were embroiled in a nasty custody dispute over their two children, a 7-year-old son and 5-year-old daughter, when Nina Reiser disappeared. Her minivan was found abandoned on Sept. 3 after she dropped the children off at her husband's house in a posh section of the Oakland hills, groceries and purse still inside.

The son, Rory Reiser, told police that he never saw his mother leave the house after dropping him off, which fits the prosecution's theory that Nina Reiser was killed there.

But during a pretrial hearing, the boy testified that he saw his mother drive away.

Neither side may get the chance to question the boy about his apparently conflicting statements after he failed to return from a holiday trip to Russia, where his maternal grandmother has begun custody proceedings.

Prosecutors have argued that, even without a body, the physical evidence they have uncovered clearly implicates Hans Reiser, a prominent software engineer, in his wife's death.

Bloodstains in Hans Reiser's house and car matched Nina Reiser's DNA, police said. Investigators searching Hans Reiser's Honda CRX after his wife's disappearance found trash bags, masking tape, absorbent towels and a floorboard soaked with water, and the car was missing its passenger seat.

They also found two books: "Homicide: A Year on the Killing Streets," by David Simon, about the Baltimore police homicide squad, and "Masterpieces of Murder," by Jonathan Goodman, about notorious murder cases.

But whatever suspicion their findings manage to cast on Reiser, prosecutors must still contend with California's tough rules on circumstantial evidence. Judges in the state are required to instruct jurors that if the evidence suggests "two reasonable interpretations," one pointing to guilt and the other innocence, they must reject the guilty interpretation.

The decision of Reiser's lawyers not to use legal tactics delay the beginning of his trial as long as possible suggests the defense feels confident it can create uncertainty among jurors, Talbot said.