What is the law governing the voluntariness of statements to police?

The law governing the voluntariness of statements to police was summarized in People v. Linton (2013) 56 Cal.4th 1146 at pages 1176-1177: " ' "A statement is involuntary if it is not the product of ' "a rational intellect and free will." ' [Citation.] The test for determining whether a confession is voluntary is whether the defendant's 'will was overborne at the time he confessed.' " ' [Citations.] [] ' "A confession may be found involuntary if extracted by threats or violence, obtained by direct or implied promises, or secured by the exertion of improper influence. [Citation.] Although coercive police activity is a necessary predicate to establish an involuntary confession, it 'does not itself compel a finding that a resulting confession is involuntary.' [Citation.] The statement and the inducement must be causally linked. [Citation.]" [Citation.]' [Citation.] A confession is not rendered involuntary by coercive police activity that is not the 'motivating cause' of the defendant's confession. [Citation.] [] 'The prosecution has the burden of establishing by a preponderance of the evidence that a defendant's confession was voluntarily made.' [Citation.] 'Whether a confession was voluntary depends upon the totality of the circumstances.' "