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Michelle Le, a 26-year-old nursing student, went missing on May 27, 2011. (Courtesy of Le family)

OAKLAND -- For more than six years Giselle Esteban was obsessed with the unfounded belief that her ex-boyfriend and father of her daughter was having an affair with her one-time best friend Michelle Le.

It was an obsession that led Esteban to make death threats against her ex-boyfriend Scott Marasigan and Le.

It was an obsession that had Esteban spending months making extraordinary efforts to find Le's home address and to scour the Internet in search of ways to kill without being caught, evidence and testimony in Esteban's trial showed.

And, it was an obsession that finally led to Esteban killing Le, 26, in a Hayward hospital parking garage. On Monday, after four days of deliberations, a jury of six women and six men decided the deliberate way in which Esteban followed through on her obsession made the 28-year-old Union City woman guilty of first-degree murder. She will go to prison for, most likely, the rest of her life.

"We were just hoping and praying the jury would make the right call, and this is it," said Le's brother, Michael Le. "We feel that a tremendous burden has been lifted."

Michael Le, his father and other family members held hands in the front row of Judge Jon Rolefson's courtroom as the jury walked in and handed over it's verdict. When the clerk read the decision, several family members began to cry.

"I know Michelle is resting in peace knowing that justice is done," said her father, Son Le. "It's just so sad to have a loss like this."

Esteban showed no emotion as the verdict was read. She sat in her chair facing forward not looking at the jury.

Le was murdered on May 27, 2011, in the parking garage of the Kaiser Medical Center in Hayward as she walked to her car to retrieve some cold medicine in the middle of her shift training as a nurse. She was initially considered a missing person and her family spent months hoping to find her alive.

But Hayward police became suspicious of Esteban after Le's disappearance when investigators learned of the threats she had made and following suspicious answers she gave to many questions during an interview at her home about 24 hours after Le disappeared.

Yet, murder charges against Esteban were not filed until September 2011 as police conducted a thorough investigation that ruled out other suspects and provided deputy district attorney Butch Ford with enough evidence to prove first-degree murder without having a cause of death.

About 10 days after Esteban was charged with murder, Le's remains were found in a secluded area near the Pleasanton-Sunol border. A cause of death could not be determined because all that remained were bones.

Ford presented the jury with much of the evidence collected including surveillance videos that showed Esteban breaking into the administrative offices of a San Mateo university Le attended and walking into an Apple store after Le's disappearance where she convinced a worker to unlock Le's cell phone, which Esteban took after killing Le.

The evidence also included conversations Marasigan, 28, secretly recorded during which Esteban threatened his life and Le's and text messages he saved in which Esteban frequently cursed Le's name and accused her of ruining her relationship with Marasigan and her family.

During closing arguments, Ford called Esteban a sociopath who had one horrible goal: to kill Le.

"She hunted down Michelle and she killed her," Ford said after the verdict was read. "I'm just glad the jury did what was right. I'm just grateful."

Esteban's attorney, Andrea Auer, tried to convince the jury that the killing was manslaughter, done in a heat of passion after a conversation Esteban had with Le in the hospital parking garage.

But Esteban chose not to testify during the trial and the only evidence Auer had of Esteban's thoughts were the hundreds of text messages she had sent to Marasigan over the years.

Auer, who left the courtroom through a back hallway and declined to comment, never disputed the evidenced presented at trial against her client but asked the jury to view it differently than the prosecutor.

Auer said the evidence showed a woman becoming increasingly paranoid and emotionally distressed as she lost custody of her child and the relationship she had with Marasigan who, less than a year earlier, was sending her sexual text messages with promises of trying to mend their broken family.

Auer said her client blamed her troubles on Le and simply wanted to have a conversation with the nursing student when she tracked Le down in the parking garage. It was a conversation, Auer said, that went horribly wrong.

Esteban now faces at least 25 years to life in prison when she is sentenced on Dec. 10.