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Imagine that you are riding the city bus to your friend's house. Someone gets on the bus after you, walks up the aisle, snatches your backpack away from you, opens it, and looks inside. If you want to sue this person, do you have a 4th Amendment constitutional claim against them?
No! Remember that the 4th Amendment protects you from unreasonable searches and seizures by members of the U.S. Government. However, you may be able to file an "invasion of privacy" lawsuit in court against that person.