Among Mr. Watson’s customers was Steven Carrillo of Winters, Calif., prosecutors said. Mr. Carrillo purchased a “Portable Wall Hanger — Single” through Mr. Watson’s website in January, they said. In May, the authorities said, Mr. Carrillo shot two security officers outside a federal courthouse in Oakland, killing one of them; in June, the authorities said, he shot two Santa Cruz County sheriff’s deputies, killing one of them.
An AR-15 rifle recovered at the Santa Cruz scene was linked to both shootings, according to federal officials. The weapon appeared “to be a Privately Made Firearm (PMF) with no manufacturer’s markings,” federal officials said in the criminal complaint against Mr. Carrillo.