Jacob Sullivan pleaded guilty to first-degree murder and other offenses for killing Grace Packer in 2016 as part of a rape-murder fantasy he shared with the teen's mother. Jurors decided in March he should get the death penalty.
The jury deliberated over parts of three days before making its decision. Jurors told the judge during the deliberations they were unable to agree, and Sullivan's lawyers argued Wednesday the judge "forced" jurors to keep deliberating, sending the message that a life sentence "was unacceptable."
Pennsylvania has a moratorium on the death penalty, but juries can still impose the sentence.
Grace Packer's mother, Sara Packer, received a life sentence in a plea deal with prosecutors.
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