Jacob Sullivan, left, and Sara Packer, right, are seen in photos released by the Bucks County District Attorney’s Office.

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A Pennsylvania man should be put to death for killing and dismembering his girlfriend’s 14-year-old daughter as part of a rape-murder fantasy he and the teen’s mother shared, a prosecutor declared Friday as he described how the victim fought for her life.

Jacob Sullivan, 46, pleaded guilty to all charges last month in a case that raised questions about the child welfare system’s failure to protect Grace Packer, who spent years in an abusive home before she was raped, drugged, bound and gagged for hours and then, finally, strangled in the attic of a suburban Philadelphia home in 2016.

Prosecutors said Grace’s adoptive mother, Sara Packer, plotted the crime with Sullivan and watched him violate and kill her daughter. Sara Packer, a former foster parent and county adoptions supervisor, agreed to plead guilty in exchange for a life sentence and is scheduled to testify at the penalty phase of Sullivan’s trial, which opened Friday outside Philadelphia. It’s expected to last several days.

In his opening statement, Bucks County District Attorney Matthew Weintraub said he would be asking the jury to impose the death penalty for Sullivan’s “awful, unspeakable, heinous crimes.” Sullivan and Packer “decided together that Grace was not worth the air she breathed,” Weintraub said.

Sullivan’s lawyer, Jack Fagan, asked jurors to spare Sullivan’s life, saying he should get the same sentence as Sara Packer. Fagan said Packer was controlling and manipulative, hated Grace long before she met Sullivan online in 2013, and masterminded the rape and murder plot.

“Sara Packer was the driving factor in the intent, the planning and the execution of what happened to her daughter,” he said.

The defense plans to call Packer as a witness.

The jury that will decide Sullivan’s sentence must be unanimous to impose the death penalty; otherwise Sullivan will get life without parole. Even if he’s sentenced to death, it’s unclear whether the punishment would ever be carried out. Democratic Gov. Tom Wolf declared a moratorium on capital punishment in 2015. Pennsylvania last carried out an execution in 1999.

Sara Packer and her husband at the time, David Packer, adopted Grace and her brother in 2007. The couple cared for dozens of foster children before David Packer was sent to prison for sexually assaulting Grace and a 15-year-old foster daughter at their Allentown home, about an hour outside Philadelphia.

Sara Packer lost her job as a Northampton County adoptions supervisor in 2010 and was barred from taking in any more foster children.

The Pennsylvania Department of Human Services launched an investigation after Grace Packer’s murder but its findings have not been made public.

On Friday, Weintraub said Grace’s short life was a series of terrible misfortunes, culminating in her agonizing death.

“What is the worst thing you can think of you can do to a child?” he asked the jury. “Because in this courtroom, we are going to check all of those boxes.”

Sullivan has admitted he punched and raped Grace, bound her hands and feet with zip ties and stuffed a ball gag in her mouth. Prosecutors said Sullivan and Packer also gave her what they intended to be a lethal dose of over-the-counter medication and left her to die in a sweltering attic.

Grace eventually managed to escape some of her bindings and spit the gag out, but was unable to make it out of the house before Sullivan and Packer returned the next day — some 12 hours later — and Sullivan strangled her.

The couple stored her body in cat litter for months, then hacked it up and dumped it in a remote area where hunters found it in October 2016, prosecutors said.

Abington Township Detective Cindy Pettinato, who investigated Grace Packer’s disappearance, testified Friday that Sara Packer seemed unconcerned her daughter was gone, quickly ridding the house of her daughter’s belongings.

Amy Adam, Grace’s guidance counselor, said the teen got excellent grades but required emotional support. She said that a few months before Grace’s death, Sara Packer had threatened to send her to foster care or to a residential center. That prompted Grace to write a “good-bye letter” to her family, Adam said.

The letter was displayed for the jury.

“I know that you guys are at your breaking point and I understand,” Grace wrote. “I love you guys I always have and I always will.”