'The ultimate issue of voluntariness' is a legal question requiring independent federal determination.' Miller v. Fenton, 474 U. S. 104, 474 U. S. 110 (1985). See also Mincey v. Arizona, 437 U. S. 385, 437 U. S. 398 (1978); Davis, 384 U.S. at 384 U. S. 741-742; Haynes, 373 U.S. at 373 U. S. 515; Chambers v. Florida, 309 U. S. 227, 309 U. S. 228-229 (1940).
'Coercion can be mental as well as physical, and . . . the blood of the accused is not the only hallmark of an unconstitutional inquisition.' Blackburn v. Alabama, 361 U. S. 199, 361 U. S. 206 (1960). See also Culombe, supra, 367 U.S. at 367 U. S. 584; Reck v. Pate, 367 U. S. 433, 367 U. S. 440-441 (1961); Rogers v. Richmond, 365 U. S. 534, 365 U. S. 540 (1961); Payne v. Arkansas, 356 U. S. 560, 356 U. S. 561 (1958); Watts v. Indiana, 338 U. S. 49, 338 U. S. 52 (1949). In Payne, the Court found that a confession was coerced because the interrogating police officer had promised that, if the accused confessed, the officer would protect ...