What is the test for probable cause in a civil forfeiture proceeding?

We review de novo the district court's determination of the existence of probable cause in a civil forfeiture proceeding. See United States v. Padilla, 888 F.2d 642, 643 (9th Cir.1989). Before a forfeiture will lie, the United States must establish probable cause that the currency was exchanged for or intended to be exchanged for drugs. See id. at 643. Probable cause means that the aggregate of facts gives rise to "more than mere suspicion." See id. at 643-44. Circumstantial evidence of drug transactions may support the establishment of probable cause. See id. at 643.