SAN DIEGO — Arsenic poisoning is difficult to diagnose.

Sgt. Todd Sommer was allegedly poisoned with the colorless, odorless, tasteless chemical in 2002. But none of his lab tests in the days before his death, nor the toxicology tests conducted postmortem, would have indicated arsenic poisoning, according to experts who testified Tuesday at the first-degree murder trial of Sommer's widow.

"If he had been dosed with 1,020 times too much arsenic ... would his labs be normal?" defense attorney Robert Udell asked Dr. Jerry Spencer, a forensic pathologist with the Armed Forces Institute of Pathology (AFIP).

"They could be," Spencer said.

Sommer's kidney tissues did reveal 1,020 times the normal levels of arsenic when they were tested for heavy metals at AFIP more than a year after his death.

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Prosecutors claim that his widow, Cynthia Sommer, a 33-year-old mother of four, killed the 23-year-old Marine for his veteran's benefits.

Their strongest evidence is Sommer's behavior following her husband's death. Two months later, Sommer had breast implant surgery and began dating another Marine. After placing 49 percent of Todd's $250,000 life insurance payout in trust funds for herself and her four children, Sommer ran though the balance in less than a year.

But, despite their continuing search into Sommer's financial, telephone, computer and library records — and an earnest analysis of her TV-viewing habits — prosecutors have been unable to link Sommer to arsenic.

An investigator admitted on the stand Tuesday that, when he traveled to London to question Sommer's ex-husband, he asked the man if Sommer ever watched "CSI" or "Forensic Files."

"He said, 'No.' That she watched 'Party of Five,' 'Beverly Hills 90210,' 'Melrose Place' and 'Friends,'" testified Naval Criminal Investigative Services Special Agent Ricky Rendon.

"Of all those shows," Rendon said, "'Melrose Place' seemed to be the most vindictive."

Rendon contacted the producers of that TV series, starring Heather Locklear, which ran from 1992-1999. He discovered that six episodes dealt with poisoning, including "spouse poisoning," Rendon said.

Prosecutors did not present any evidence that Sommer mimicked those episodes.

Jurors also squinted to view Rendon's underdeveloped photos, projected on a screen Tuesday, depicting boxes of "Grant's Kills Ants."

Rendon said he purchased the arsenic-laced ant poison at the base grocery store, the same store where the defendant would likely have shopped.

Rendon did not obtain purchasing records to indicate whether Sommer ever bought Grant's Kills Ants.

Sommer has pleaded not guilty to first-degree murder and the special circumstances of murder for financial gain by the administration of poison.

She wore a brown pantsuit, with her long brown hair in a French braid, and whispered to her attorney's legal assistant in court Tuesday. If she takes the stand in her own defense this week, she is expected to tell the jury that she loved Todd Sommer deeply and had nothing to do with his mysterious death.

And while a series of prosecution witnesses have testified about how easy it would have been for her to buy arsenic, prosecutors have no paper or electronic trail indicating the defendant ever sought out, discussed or had access to the lethal poison.