In the United States v. Jacobsen et al., 1983, the Supreme Court ruled that a search conducted by private individuals does not constitute a Fourth Amendment violation unless they act as agents of the government or participate in governmental action. The case arose when Federal Express employees opened a damaged package and found plastic bags containing white powder. They contacted law enforcement who confirmed it was cocaine without obtaining a warrant first. The defendants argued this constituted an illegal search under the Fourth Amendment rights against unreasonable searches and seizures. The court held that since FedEx employees were acting as private parties, their initial inspection did not violate any constitutional protections; therefore, police only needed to ensure what had already been discovered by these private parties - which didn't require further invasion of privacy than what had occurred initially - hence no need for warrants before confirming contents of packages.

In the dissenting opinion for United States v. Jacobsen et al., Justice Brennan, joined by Justices Marshall and Blackmun, argued that the Fourth Amendment was violated when law enforcement agents conducted a search without obtaining a warrant first. The dissenters believed that once private freight carrier employees had opened the package and identified white powder inside it, any further examination of its contents by law enforcement constituted a separate search which required an independent justification. They disagreed with the majority's view that because private individuals had already invaded Jacobson’s privacy, no constitutional violation occurred when federal agents repeated this invasion. Instead, they contended that each new intrusion into someone's privacy constitutes a fresh violation of their rights unless justified under one of the established exceptions to warrant requirements - none of which applied in this case according to them.