The mere fact that someone is under suspicion for a crime is not enough to justify a warrant for that person's cellphone. The mere fact that most people have a cellphone is not enough to justify a warrant. A warrant issued with no more specific reason than those is unconstitutional, the influential D.C. Circuit Court of Appeals ruled recently.
Although the D.C. Circuit doesn't include Wisconsin, the D.C. Circuit Court of Appeals is considered quite influential and has jurisdiction over much of what federal agencies do, and other federal courts will give weight to its decisions in their own cases.
The case before the appellate court involved a man who was convicted of being a felon in possession of a gun, which is prohibited. He was not initially suspected of possessing a weapon, however, but was thought to be involved as a driver in a separate crime.
In 2013, a judge issued a warrant which allowed the D.C. police to enter the man's residence and seize any cellphones or electronic devices found there. When officers arrived at the residence, a gun was thrown from a window. They seized that gun and charged the defendant.
The problem was apparently that the search warrant wasn't supported by any evidence that the man owned a cellphone or other electronic devices, or that those devices contained anything incriminating. The police who applied for the warrant thought it was enough to assume he had a cellphone because most people do, and to speculate that his cellphone probably contained incriminating information.
An overly broad warrant is often referred to as a "fishing expedition." In order to obtain a search warrant, law enforcement is supposed to persuade a judge that they have probable cause to suspect that criminal activity or evidence of a crime will be found in the location to be searched. It cannot be based on guesswork, hunches or bias. The Fourth Amendment requires more than mere suspicion.
Judges ideally should recognize when a warrant is unsupported by probable cause and therefore decline to authorize a search warrant, but that doesn't always happen. Whenever a search is found to be based on insufficient legal justification, however, it violates the constitutional rights of the defendant. The government is not allowed to benefit from violating people's constitutional rights, so anything found during an unlawful search cannot be used in court.
In this case, the appeals court ruled that the weapon could not be used as evidence. Without that, the prosecution will likely have to dismiss the case entirely.