What is the test for an involuntary confession under the Fourteenth Amendment?

Clause of the Fourteenth Amendment." (Colorado v. Connelly (1986) 479 U.S. 157, 167 [93 L.Ed.2d 473, 484].) Coercive police activity, however, " 'does not itself compel a finding that a resulting confession is involuntary.' [Citation.] The statement and the inducement must be causally linked. [Citation.]" (People v. Maury (2003) 30 Cal.4th 342, 404-405; see also People v. Williams (1997) 16 Cal.4th 635, 659 ["confession or admission is involuntary, and thus subject to exclusion at trial, only if it is the product of coercive police activity"].)