SAN DIEGO -- A toxicologist gave her husband an obscure drug to kill him because he was about to expose her methamphetamine use and affair with her supervisor, a prosecutor told jurors Tuesday in the young woman's murder trial.

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Deputy District Attorney Dan Goldstein said Kristin Rossum, 25, killed Gregory de Villers with the help of her Australia-born lover, Michael Robertson.

"Michael Robertson will never be in this courtroom," Goldstein told the seven men and five women. "He protected her in every way, including trying to cover up this homicide."

In court papers, Goldstein referred to Robertson, the chief toxicologist at the San Diego County Medical Examiner's Office at the time, as an "unindicted co-conspirator."

Robertson moved back to Australia after de Villers' death on Nov. 6, 2000.

During a four-hour opening statement, Goldstein said Rossum (pictured, right, in courtroom sketch) killed her 26-year-old spouse by stealing and apparently injecting him with fentanyl, which is said to be 100 times stronger than morphine.

Besides fentanyl, the painkiller Oxycodone and the sedative Clonazepam were found in de Villers' system. No syringes or fentanyl patches were near his body, but there were three needle marks on his arm, authorities said.

"Greg de Villers was going to reveal her stealing methamphetamine and having an affair and she wasn't going to let him do it," the prosecutor said.

When questioned by police, Rossum denied using drugs and denied the affair with Robertson, Goldstein said.

He said de Villers detested drugs and was the one responsible for getting his wife of 17 months off meth.

De Villers' death was first reported as a suicide, but Rossum was arrested eight months later and charged with his murder.

"He had no reason to take his own life," Goldstein told the jury.

Rossum used the "tools of her trade" in the county Medical Examiner's Office to poison her spouse, the prosecutor said.

The trial -- expected to last three to four weeks -- reads like a murder mystery with sex, drugs, affairs and more, Goldstein told the jury.

"All roads are going to lead to the defendant's guilt," the prosecutor said.

Rossum grew up in a well-to-do family in Los Angeles, the prosecutor said, and was the "apple of her father's eye" until she turned to drugs in the 1990s. Her drug use "started to cause enormous stress in her family," Goldstein said.

Ralph Rossum is a college professor who worked for the Reagan administration in the 1980s. Constance Rossum is also a professional woman, Goldstein said. Both worked to shield their daughter from trouble, the prosecutor said.

"To Kristin Rossum and the Rossum family, image is everything," Goldstein said.

The defendant ran away from home and was not in college after graduating from high school, the prosecutor said.

"We had a horrendous time," Constance Rossum wrote.

"Where did we go wrong? How do we explain this to anybody?" Ralph Rossum wrote of his daughter's problems before she met Greg de Villers.

Kristin Rossum moved to San Diego and tried to get a job with the Sheriff's Department but was rejected because of her past drug use, the prosecutor said.

But the county Medical Examiner's Office failed to do a background check on Rossum and she was hired, Goldstein said.

In March 2000, the defendant began her affair with Robertson, the prosecutor said.

He displayed a number of e-mails Rossum and Robertson exchanged.

"I love you with all I have to give," Rossum wrote Robertson.

"All I want to do is spend the rest of my life with you," Robertson wrote back.

"When I see you, I see my future," Rossum wrote in another e-mail to Robertson.

Prosecutors allege Rossum staged an "American Beauty" suicide scene to make it look as though her husband took his life.

A witness will testify that the movie is one of Rossum's favorite films, the prosecutor said.

Rossum told detectives that she checked on the victim a number of times the day he died.

She said he was cold to the touch when she felt him sometime after 9 p.m., Goldstein told the jury.

"I pulled back the covers and there were rose petals all over him," the prosecutor said of the defendant's statement to law enforcement.

The first paramedic on the scene, Sean Jordan, testified as the trial's first witness that there were no rose petals in the bed. He said they were strewn all over the floor next to the victim's body.

Rossum told a friend she didn't know where the rose came from, but a Vons receipt showed the defendant bought a single rose just after noon that same day, Goldstein told the jury.

"She bought a rose at 12:41 p.m., while her husband was dying," the prosecutor said. "One single rose."

The rose, a fabricated journal and a shredded love note from Robertson made up a "staged scene," Goldstein said.

The items that were found "are tantamount to a confession," the prosecutor told the jury.

Rossum wept as her attorney, Alex Loebig, told the jury she would testify about her life with Greg de Villers.

"You're going to get it firsthand," her attorney said.

He suggested that de Villers committed suicide because he didn't want a life without Rossum.

The victim told his wife a number of times that he "didn't want to live without her," Loebig told the jury.

The attorney took issue with the characterization of the Rossum family.

"This is not a blue-blood family with a black sheep in it," the attorney said.

He said the defendant had her share of problems in high school after a girlfriend introduced her to methamphetamine.

Rossum graduated with honors from San Diego State University after de Villers helped get her off drugs, Loebig said. De Villers pressed hard to get married but Rossum questioned the wisdom of such a union, the attorney said.

"I don't know, I'm so young," Rossum said as the marriage grew closer, Loebig told the jury.

De Villers was ecstatic about marrying Rossum but the feeling was not reciprocal, Loebig said.

Soon after they were married, Rossum suggested a trial separation and counseling, but de Villers reacted with anger and brooding, Loebig said.

As for Robertson, the lawyer said, the defendant had no control over who would be her boss at the Medical Examiner's Office. Robertson was a big, Australian rugby player who landed in Rossum's life, the attorney said.

"He was one smooth guy," the attorney said.

A lot of people in the office knew about the affair between Rossum and Robertson, Loebig said. Rossum knew others in the Medical Examiner's Office who had had affairs, the attorney added.

"So where's the motive in that?" he asked.

The day he died, de Villers told his brother he was drinking gin, which was unusual, Loebig said. A big green cup with clear liquid was discovered by the bed where de Villers died, the attorney said.

"There is no telling if fentanyl was in there," Loebig told the jury.

He said an expert will testify that the powerful drug was ingested.

Rossum didn't kill her husband for money, because they were struggling financially, Loebig said.