Petitioner, a Bureau of Alcohol, Tobacco and Firearms agent, prepared and signed an application for a warrant to search respondents' Montana ranch, which stated that the search was for specified weapons, explosives, and records. The application was supported by petitioner's detailed affidavit setting forth his basis for believing that such items were on the ranch and was accompanied by a warrant form that he completed. The Magistrate Judge (Magistrate) signed the warrant form even though it did not identify any of the items that petitioner intended to seize. The portion calling for a description of the "person or property" described respondents' house, not the alleged weapons; the warrant did not incorporate by reference the application's itemized list. Petitioner led federal and local law enforcement officers to the ranch the next day but found no illegal weapons or explosives. Petitioner left a copy of the warrant, but not the application, with respondents. Respondents sued petitioner and others under Bivens v. Six Unknown Fed. Narcotics Agents, 403 U. S. 388, and 42 U. S. C. §1983, claiming, inter alia, a Fourth Amendment violation. The District Court granted the defendants summary judgment, finding no Fourth Amendment violation, and finding that even if such a violation occurred, the defendants were entitled to qualified immunity. The Ninth Circuit affirmed except as to the Fourth Amendment claim against petitioner, holding that the warrant was invalid because it did not describe with particularity the place to be searched and the items to be seized. The court also concluded that United States v. Leon, 468 U. S. 897, precluded qualified immunity for petitioner because he was the leader of a search who did not read the warrant and satisfy himself that he understood its scope and limitations and that it was not obviously defective.

Held:

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