Sattazahn (D) and his accomplice, Jeffrey Hammer, hid in a wooded area waiting to rob Richard Boyer, manager of the Heidelberg Family Restaurant. D carried a .22-caliber Ruger semiautomatic pistol and Hammer a .41-caliber revolver. They accosted Boyer in the restaurant's parking lot at closing time. With guns drawn, they demanded the bank deposit bag containing the day's receipts. Boyer threw the bag toward the roof of the restaurant. D commanded Boyer to retrieve the bag, but instead of complying Boyer tried to run away. Both D and Hammer fired shots, and Boyer fell dead. The two men then grabbed the deposit bag and fled. D was convicted of first-, second-, and third-degree murder, and various other charges. The Commonwealth presented evidence of one statutory aggravating circumstance: commission of the murder while in the perpetration of a felony. D presented as mitigating circumstances his lack of a significant history of prior criminal convictions and his age at the time of the crime. Pennsylvania law requires that for a death sentence the jury must unanimously find at least one aggravating circumstance ... and no mitigating circumstance or if the jury unanimously finds one or more aggravating circumstances, which outweigh any mitigating circumstances. The verdict must be a sentence of life imprisonment in all other cases. The jury was deadlocked at 9 to 3 for life imprisonment. D moved that the jury be discharged and that [the court] enter a sentence of life imprisonment.' The trial judge, in accordance with Pennsylvania law, discharged the jury as hung and indicated that he would enter the required life sentence. D appealed to the Pennsylvania Superior Court. That court concluded that the trial judge had erred in instructing the jury in connection with various offenses with which petitioner was charged, including first-degree murder. It accordingly reversed D's first-degree murder conviction and remanded for a new trial. Pennsylvania filed a notice of intent to seek the death penalty. In addition to the aggravating circumstance alleged at the first sentencing hearing, the notice also alleged a second aggravating circumstance, petitioner's significant history of felony convictions involving the use or threat of violence to the person. (This was based on guilty pleas to a murder, multiple burglaries, and a robbery entered after the first trial.) D moved to prevent Pennsylvania from seeking the death penalty and from adding the second aggravating circumstance on retrial. The trial court denied the motion, the Superior Court affirmed the denial, and the Pennsylvania Supreme Court declined to review the ruling. At the second trial, the jury again convicted petitioner of first-degree murder, but this time imposed a sentence of death. D appealed again on double jeopardy grounds. The Pennsylvania Supreme Court concluded that neither the Double Jeopardy Clause nor the Due Process Clause barred Pennsylvania from seeking the death penalty at D's retrial.