(CNN) -- Calling capital punishment in California an "empty promise," the father of murdered teen Chelsea King said he supported a deal to take death off the table for his daughter's killer in order to bring closure to the community.
"We stand here because of a despicable evil act committed against our beautiful daughter, Chelsea, committed against our family and committed against our community," Brent King said in a news conference Friday. "While our unequivocal first choice is the death penalty, we acknowledge that in California that penalty has become an empty promise."
San Diego County District Attorney Bonnie Dumanis said the family's blessing contributed to a plea deal with registered sex offender John Gardner III in Chelsea's death and two other cases.
Gardner, 31, pleaded guilty Friday to the murder of King, the murder of Amber Dubois, and assault with intent to commit rape of a third person.
King, 17, was last seen alive leaving Poway High School in suburban San Diego on February 25. Her car, with her cell phone inside, was found at Rancho Bernardo Community Park, where she was known to run on the trails. Her disappearance sparked a massive search that ended a few days later with the discovery of her remains in the park. Dubois, 14, disappeared in February 2009 while walking to school in Escondido. She was considered a missing person for more than a year, until her remains were found in March.
Prosecutors revealed in court Friday that Gardner led authorities to Dubois' body in exchange for assurances that it would not be used against him in court.
In exchange for his guilty pleas, Gardner is to be sentenced to two consecutive terms of life without the possibility of parole. He also waived his right to appeals, ensuring that he will die in prison, Dumanis said at the news conference.
Superior Court Judge David Danielsen accepted the plea and scheduled sentencing for June 1. A gag order is in place until then.
The surprise change of plea came during a hearing Friday, after prosecutors charged him with murder with a special circumstance of rape for Dubois's death.
Gardner was facing the death penalty on one charge of murder with a special circumstance of rape for King's death.
Dressed in dark blue jailhouse garb, his wrists chained, Gardner nervously responded "yes" several times to the judge's questioning of whether he understood his rights and was entering his plea willingly. He cast his gaze downward as the judge read brief descriptions of how he raped and murdered Dubois and King. Gardner made no statement.
According to court documents, on February 13, 2009, Gardner abducted Dubois and brought her to a remote area of Pala, where he raped and stabbed her, and buried her in a shallow grave.
Gardner admitted to attacking King while she was running and dragging her to a remote area, the documents said. He raped and strangled her, and also buried her body in a shallow grave.
Gardner also admitted to attacking another female on December 27, 2009, while she was running.
In a news conference after Friday's hearing, Dubois' father, Moe, expressed gratitude over the case's resolution.
"As you can imagine, this turn of events in the case came as a surprise to all of us when we were informed about the details yesterday," he said. "As a parent I am thankful [for] the work put forth by the district attorney's office and the defense team in coming forth with a resolution in the case and allowing us to have justice and closure for Amber's case."
Without the plea deal, Dumanis said her office did not have enough evidence to charge Gardner in the death of Dubois.
Her remains were not found until Gardner led authorities to her body, three days after he was charged in King's death, Dumanis said.
"The only promise made to him in exchange for this information was that we could not use it against him in court. This was a somber decision," the district attorney said in a press conference after Friday's hearing, surrounded by tearful family members. "To end the anguish of the unknown for the Dubois family and to bring Amber home, we agreed we would not use this information against Gardner in court."
Further efforts to collect evidence linking Gardner to Dubois' death were unsuccessful, Dumanis said, leaving her office otherwise unable to pursue charges against him -- until he offered to plead guilty.
"Accepting this plea has been an extremely difficult decision. We have the evidence to pursue a murder charge against the defendant for Chelsea's murder, but not for Amber's murder," she said.
"By accepting this guilty plea, we are obtaining a conviction for the murder of Amber that we otherwise would not have been able to obtain."
The prosecutor also echoed the sentiments of King's father on the death penalty in California, where 13 executions have been carried out since the U.S. Supreme Court reinstated capital punishment in 1976.
"Most of us realize a death sentence at this time is a hollow promise in California. Even if death was imposed, Brent, Kelly and their family would have to endure a preliminary hearing, a trial, decades of appeals and the pain of reliving the murder over and over again," she said. "In addition, as parents they realized what Amber's parents were facing. Her case would have no legal closure."
A resolution for the Dubois family also figured into the King family's decision to support the plea, Brent King said.
"We find ourselves in a position to help give another grieving family a measure of closure. The Dubois family has been through unthinkable hell the past 14 months. We couldn't imagine the confession to Amber's murder never seeing the light of day, leaving an eternal question mark," he said.
"There's nothing, nothing satisfying about this moment. It's only one more unbearably painful day that we'll have to carry in our memory as long as we live."