Randall Fields, in prison on a disorderly conduct conviction, was escorted into a conference room where he was questioned by two sheriff's deputies for approximately seven hours concerning an allegation of sexual conduct with a minor.
According to the Supreme Court, he was told he was free to leave and return to his cell at least twice, but the deputies were armed during the interrogation. Fields confessed during the interrogation and testimony from the trial indicated that while he "no longer wanted to talk to the deputies, but he did not ask to return to his cell."
Custody?
Justice Alito, writing for the majority, found Fields was not in custody, because he was told he could leave at any time. Alito spends much of his opinion parsing closely the language of Miranda and later precedent to find justification for the lack of the Miranda warnings.
The fact he was in prison means he was not "shocked" by being isolated and questioned by deputies in the same way an ordinary citizen might be. Further, because they are in prison, the "inherently compelling pressures" that a citizen off the street might feel are absent. After the questioning, they know they are going back to a cell.
Justice Alito also notes a prisoner knows the officers have limited authority to "free" him and they know they are already incarcerated, and no matter what they say, they will not go free at the end of the questioning. Because the conditions that gave rise to the necessity of the Miranda rights are not present here, he finds no violation of the Fifth Amendment.