What is the burden of establishing that secondary evidence was tainted by the police?

"As for secondary evidence, the defendant bears the burden of making a prima facie case that such evidence was 'tainted' by--i.e., causally linked to--the primary illegality." (People v. Williams, supra, 45 Cal.3d 1268, 1300, 248 Cal.Rptr. 834, 756 P.2d 221.) To do this, the defendant "must show more than that the challenged evidence 'would not have come to light but for the illegal actions of the police'; rather, [the defendant] must establish that it ' "has been come at by exploitation of that illegality...." ' " (Ibid., original italics.)