Richard Joseph Hirschfield's defense lawyers on Thursday strongly challenged the meaning and importance of the DNA evidence that prosecutors say put the 63-year-old defendant at the scene of rape and murder.

Facing odds of 240 trillion to one, the defendant's attorneys questioned the veracity of the testing that established the overwhelming likelihood that the genetic material found on a semen-stained blanket linked to the killings belonged to Hirschfield.

But if Hirschfield's Sacramento Superior Court jury chooses to believe it was his DNA, his lawyers in their closing arguments offered other explanations on how it got on a blanket in the van belonging to John Riggins when he and his girlfriend, Sabrina Gonsalves, were slain.

The defense lawyers said it's possible Hirschfield had consensual sex with somebody in the van after the killings. Or that he masturbated on it. Or that his late brother, who killed himself the day after investigators questioned him in the case, came across the van after the bludgeon-and-slash attacks on Riggins and Gonsalves, removed the blanket, brought it home, somehow touched it to a surface that contained Hirschfield's semen, and then returned it to the van.

The whole idea was to create reasonable doubt in at least one juror's mind before the case goes to the seven-man, five-woman panel, possibly as soon as today. In that effort, the defense argued there is no proof the blanket in the van had anything to do with the killings, and that if Hirschfield or his bodily fluids came in contact with it, it didn't really mean anything.

Defense attorney Linda Parisi reminded the jury of one witness who testified he saw somebody sleeping in the van the night after the killings, but before authorities found the vehicle and discovered the bodies. She raised the possibility that it was Hirschfield who had dozed off in in the vehicle just used in a heinous double murder, and that somehow his DNA got left behind.

"Is it speculation to say, is it possible, that that was a night when Mr. Hirschfield was at that location?" Parisi said. "Is it possible that he, like others, went into that van? It it possible that that's who's slumped over the wheel, sleeping in that van? Is that unreasonable with the evidence? No."

Hirschfield, 63, is charged in the Dec. 20, 1980, killings of Riggins and Gonsalves, two 18-year-old UC Davis students who were abducted after working on a performance of The Davis Children's Nutracker at the city's Veterans Memorial Theater. Their bodies were found two days later in a ravine 30 miles to the east, near Lake Natoma.

Prosecutors filed special- circumstance allegations against Hirschfield of multiple murders and murder during the course of kidnap, rape and oral copulation.

If the jury convicts him and finds the allegations true, the case will then go to the penalty phase where he will be sentenced either to death or to life in prison with no chance of parole.

In her closing argument Wednesday, Deputy District Attorney Dawn Bladet said Hirschfield – just out on parole after serving five years in prison for a 1975 sex crime in Mountain View – killed Riggins and Gonsalves to eliminate the witnesses after he abducted them and raped her.

Assistant Public Defender David Lynch, his office's DNA expert, took on the challenge of contesting the prosecution's scientific evidence. He suggested the state Department of Justice criminalists who lifted the DNA from the blanket were biased because "they all chose to become prosecution lab workers."

Lynch said the DNA may have been contaminated in the 11 years it resided in a torn evidence bag before the DOJ employees obtained the profile that excluded and exonerated four defendants previously charged in Yolo County.

He also wondered if there had been any "shenanigans" in the testing that produced the profile, saying one private criminalist who had access to the blanket once worked for the Yolo defendants.

As for the one-in-240 trillion match the DOJ criminalists established between the DNA found on the blanket and Hirschfield's reference profile, Lynch told the jury, "if that's something you're convinced of," then there are other reasonable scenarios of how it got there. Among them, he said, were the possibilities of consensual sex, masturbation and Hirschfield's brother's removal-and-return of the blanket.

Lynch never argued directly that the DNA was not Hirschfield's. But he insisted the "minor donor" in a mixed sample on the blanket was not from the saliva of Sabrina Gonsalves, as alleged by the prosecution.

In her presentation, Parisi focused on the Yolo foursome that was cleared in 1993 – David Hunt, his wife Suellen Hunt, Richard Thompson and Douglas Lainer. She asked jurors to believe the testimony from a paid police informant who said that Thompson, now dead, admitted the killings to him.

She also cited a statement given by a former girlfriend, also dead, of Thompson's who said he explained to her the killings took place because Riggins and Gonsalves were witnesses against Hunt's half-brother, the late convicted serial sex killer Gerald Gallego.

Parisi also said that assorted sightings of Riggins' van after the abduction and the location of the bodies showed that whoever did the killings wanted them discovered quickly, so authorities might be drawn off Gallego, who had recently been jailed on his murder spree.

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