The purchase of a chemical that could be used to make a toxic chemical weapon, legal in itself, can be probable cause. “A series of legal acts may provide the basis for probable cause of illegal acts.” United States v. Michell, 2018 U.S. Dist. LEXIS 94318 (D. Ariz. June 5, 2018):
… Mr. Michell argues that because the mere purchase and possession of potassium cyanide is not a crime, the government lacked probable cause that Mr. Michell would use the potassium cyanide as a chemical weapon.
A series of legal acts may provide the basis for probable cause of illegal acts. As the Ninth Circuit has described, innocent acts, which “if reviewed separately, might be consistent with innocence,” may establish probable cause if “considered as a totality, would likely create a fair probability of suspicion in any person of average acumen and experience.” United States v. Ocampo, 937 F.2d 485, 490 (9th Cir. 1991) (quoting United States v. Hoyos, 892 F.2d 1387, 1393 (9th Cir. 1989)).