The Atlantic; December 30, 2013
Many Americans reacted with outrage when they learned that the NSA stores details about phone calls made by virtually everyone in the United States. They felt a strong, if vague, notion that the practice must violate their constitutional rights. Couldn't NSA analysis of telephone metadata reveal sensitive, private details about most anyone in the country, like their network of friends, the identity of their sexual partners, or their contact with medical or mental health professionals? Aren't mass searches of innocents anathema to the Fourth Amendment?
The legal response from NSA defenders has leaned heavily on the precedent set in Smith v. Maryland, a Supreme Court case decided in 1979, before the era of big data.
The case concerned a robbery. Patricia McDonough, the victim, noticed a 1975 Monte Carlo near the scene of the crime. Later she received obscene phone calls from a man who claimed to be the robber. Once he asked the victim to step out onto her porch, where she saw the Monte Carlo drive slowly by. Soon after, police spotted a man driving the Monte Carlo in the victim's neighborhood. After tracing its license plate to Michael Lee Smith, officers went to the phone company and asked them to put a pen register on his phone. The resulting phone records showed that he called the victim's house. At trial, he tried to suppress that evidence, arguing that the police should have gotten a warrant before having the phone company track the numbers he dialed.
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