In what circumstances must a federal court review the voluntariness of a confession?

In this circuit, a federal court must conduct an independent review of validity of a waiver. See Grooms v. Keeney, 826 F.2d 883, 887 (9th Cir.1987). In Grooms, we stated in reference to a waiver issue:

A federal court must conduct an independent review of the voluntariness of a confession. Miller v. Fenton, [474 U.S. 104], 106 S.Ct. at 451 [88 L.Ed.2d 405 (1985) ]. In this case, the magistrate reviewed the trial transcript and transcription of the tape of the interrogation and concluded Grooms made a voluntary, intelligent and knowing waiver of his rights. We agree.

Id. ²