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.
A homicide charge against a person is one of the most severe in criminal law. To be arrested for homicide, or any other criminal charge, the police must have probable cause, a legal requirement of sufficient reason from the facts and circumstances of the situation, that an individual has committed the crime. In general, the police must have probable cause to obtain a search warrant before searching for evidence.