The United States Supreme Court has held that the prosecution at a voluntariness hearing to determine the admissibility of a challenged confession must prove voluntariness at least by a preponderance of the evidence, and that a stricter standard of proof is unnecessary.
Lego v. Twomey, 404 U.S. 477 (1972).
Paul R. Marks,
Constitutional Law - Burden of Proof at Voluntariness Hearing to Determine the Admissibility of a Confession,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol11/iss1/15