Nick Ut, File, Associated Press
FILE - In this July 29, 2009 file photo, former Los Angeles police officer Stephanie Lazarus appears in court in Los Angeles. Lazarus is accused of killing an ex-boyfriend's wife 23 years ago when she was a young officer. Opening statements in Lazarus' trial begin Monday, Feb. 6, 2012.

LOS ANGELES — Former police detective Stephanie Lazarus was either a heartbroken, jilted lover driven to kill her romantic rival, or she was an innocent woman snared in a web of mishandled scientific evidence.

The contrasting portraits emerged Monday during opening statements at the murder trial of Lazarus.

Both sides said the proof of the cold case, which lingered for 26 years, would be in the forensic evidence. The science of DNA, developed in the years after the killing, could hold the key.

"A bite, a bullet, a gun barrel and a broken heart. That's the evidence that will prove the defendant Stephanie Lazarus murdered Sherri Rasmussen," Deputy District Attorney Shannon Presby told jurors in the crowded courtroom.

He said a bite on Rasmussen's arm contained saliva that was matched to Lazarus' DNA more than two decades after the killing.

Lazarus' lawyer, Mark Overland, said the DNA material was corrupted over the decades, and a forensic dentist would testify that the bite mark does not match an impression taken of Lazarus' teeth.

Presby said it has been a long time since Rasmussen died. Ronald Reagan was then president, and Bill Cosby was the biggest star on TV. DNA didn't exist as a forensic tool but it does now, he said.

Presby acknowledged the gun used in the killing wasn't found but said the bullets came from the same type of weapon that was sold to Lazarus at the Los Angeles Police Department gun shop when she became an officer.

Lazarus, 51, looked down and took notes but showed no reaction to the opening statements.

Her lawyer tracked the course of her accomplishments in the LAPD and suggested police were correct when they initially suspected Rasmussen had been killed by burglars.

Presby told a different story. He said Lazarus was obsessed with her former boyfriend, John Ruetten, who did not return her feelings. When he told her that he was marrying another woman, she was driven to vengeance, Presby suggested.

He said she broke into the townhouse that Ruetten shared with Rasmussen, his new wife, then bludgeoned and shot her to death after a confrontation that left blood on the walls.

Presby read jurors an excerpt of a letter Lazarus wrote to her mother a few months before the killing.

"I'm totally in love with John and the past year has really torn me up," it said. "I don't think I'll ever understand his decision."

The prosecutor also showed jurors a photograph of the dead Rasmussen juxtaposed with a picture of her in her bridal gown.

"Sherri Rasmussen was wearing the gown that Stephanie Lazarus believed was hers," Presby said.

Overland said the prosecution had it all wrong. He said the relationship between Lazarus and Ruetten was not one-sided. They were college friends who later became lovers, and their sexual relationship went on for about a year, he said. She became close to his family.

"The evidence will show that Stephanie Lazarus did love him and that they had a future together," the lawyer said. "She wasn't obsessed with him."

A short time before the wedding, Lazarus appealed to Ruetten to meet with her, Overland said. He came to her home and the meeting ended with them having sex, the lawyer said.

"He told Stephanie Lazarus he was moving on with his life," the attorney said.

Lazarus subsequently confronted Rasmussen at the hospital where she worked as a nursing supervisor, Overland acknowledged. What was said at that meeting will be a subject of testimony.

The former detective has been in custody since 2009.