What is the test for a finding that probable cause exists to issue a search warrant?

We review a magistrate's determination that probable cause exists to issue a search warrant independently of the conclusion reached by the district court. United States v. Castillo, 866 F.2d 1071, 1076 (9th Cir.1988). We review the magistrate's determination that probable cause existed to issue a search warrant to see whether "under the totality of the circumstances, 'the magistrate had a "substantial basis for ... conclud[ing]" that probable cause existed.' " Id. (quoting Illinois v. Gates, 462 U.S. 213, 238-39 (1983)). We will not reverse a magistrate's finding of probable cause unless it is clearly erroneous. Castillo, 866 F.2d at 1076.