The prosecution put on what may be its most important evidence in the Richard Joseph Hirschfield murder trial Tuesday when a state Department of Justice expert testified that the odds of the defendant's DNA matching a key piece of evidence linked to the crime were one in 240 trillion.

All 13 of Hirschfield's genetic markers came up as a match to the DNA extracted from a semen-stained blanket found in a van that UC Davis students John Riggins and Sabrina Gonsalves had been driving in the night of their Dec. 20, 1980, disappearance from the college town.

"This," testified supervising criminalist Steven Myers, "is strong evidence that Mr. Richard Hirschfield is the semen donor."

Myers took the witness stand as the fifth week of Hirschfield's murder trial got under way in Sacramento Superior Court. He is charged with the murders of Riggins and Gonsalves, both 18, whose slashed and bludgeoned bodies were found two days after their disappearance in a ditch off Folsom Boulevard in Sacramento.

Hirschfield, 63, also is accused of raping Gonsalves. He faces a possible death penalty if he is convicted.

Previous evidence introduced by the prosecution at trial established Hirschfield's connections to Davis and to the area where the students' bodies were found. It also established his propensity to commit sexually based offenses, and it even had something of a confession by his brother. Joseph Hirschfield put himself at the scene of the killings in a suicide note he wrote to his wife in November 2002, a day after Sacramento County sheriff's detectives paid him a visit and told him they were investigating his possible involvement in the Riggins and Gonsalves killings.

Witnesses also said they saw a van matching the description of Riggins' vehicle in the area where the bodies were found the night they turned up missing. It was a blanket found inside that van that contained four semen stains that Myers linked to Hirschfield on Tuesday.

Myers also testified his tests on the blanket detected the presence of another "minor contributor," whose DNA was mixed in the semen stain. He identified that DNA as in all likelihood coming from Sabrina Gonsalves.

Myers said Gonsalves's genetic material probably was deposited through blood, saliva or vaginal tissue.

In his cross-examination of Myers, Assistant Public Defender David Lynch got the witness to agree his testing could not determine when or how the stains were deposited on the blanket. Myers also agreed with Lynch it is possible that DNA can be spread around by contamination, and that the criminalist couldn't say which bodily fluids were used to transmit the evidence that came from the minor contributor.

Lynch is scheduled to complete his cross-examination today, in front of Judge Michael W. Sweet. When he's finished, Deputy District Attorney Dawn Bladet will push toward the conclusion of her case with coroners' testimony on the autopsies of Gonsalves and Riggins. Bladet is expected to rest her case Thursday.

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