We know that law enforcement's seizure of a person may not be longer than necessary to effectuate the legitimate purpose of the seizure (for instance, a traffic stop). The Fourth Circuit made it clear earlier this year that the same rule applies to law enforcement's seizure of a person's cellphone (or other property in which a person has a possessory interest).
In United States v. Pratt, 915 F.3d 266 (4th Cir. 2019), the FBI was investigating the defendant for running a prostitution ring involving minors. An agent seized his cellphone without his consent. But the agent did not get a warrant to search the phone until 31 days later. At a hearing on the defendant's motion to suppress the fruit of the warrant, the defendant argued that this delay was unreasonable. The district court disagreed, but the Fourth Circuit reversed.
Absent the rightful possessor's consent, an extended seizure of property may become unreasonable, explained the Court. Here, the government's only explanation for the delay in seeking a warrant was that law enforcement had to decide in which state to seek the warrant. This explanation was insufficient: "the agents here failed to exercise diligence by spending a whole month debating where to get a warrant."
The Court also rejected the government's argument that it could hold the phone as an instrumentality of the defendant's crime. Only the phone's files had evidentiary value, and thus law enforcement could have copied the files and returned the phone.