When a suspect is arrested without a warrant, can the police delay making a probable cause determination?

When a suspect is arrested without a warrant, "a policeman's on-the-scene assessment of probable cause provides legal justification for arresting a person suspected of crime, and for a brief period of detention to take the administrative steps incident to arrest." Gerstein v. Pugh , 420 U.S. 103, 11314, 95 S.Ct. 854, 43 L.Ed.2d 54 (1975). In order to hold a suspect beyond the brief period required for administrative steps, "the Fourth Amendment requires a judicial determination of probable cause." Id. at 114, 95 S.Ct. 854.

The "Fourth Amendment permits a reasonable postponement of a probable cause determination while the police cope with the everyday problems of processing suspects through an overly burdened criminal justice system." Cnty. of Riverside v. McLaughlin , 500 U.S. 44, 55, 111 S.Ct. 1661, 114 L.Ed.2d 49 (1991). A "jurisdiction that provides judicial determinations of probable cause within 48 hours of arrest will, as a general matter, comply with the promptness requirement." Id. at 56, 111 S.Ct. 1661. Thus, when the police obtain a probable cause determination within the 48-hour window, the burden is on the plaintiff to prove that the determination was delayed unreasonably. Id. "Examples of unreasonable delay are delays for the purpose of gathering additional evidence to justify the arrest, a delay motivated by ill will against the arrested individual, or delay for delay's sake." Id.

While it is unreasonable to delay a probable cause determination to gather evidence justifying the arrest, the Fourth Amendment "does not prohibit the police from investigating a suspect while the suspect is legally detained." Kanekoa v. City & Cnty. of Honolulu , 879 F.2d 607, 612 (9th Cir. 1989). The key distinction is whether the investigation is meant to gather probable cause to justify the arrest. "[I]t is inimical to the fourth amendment for the police to arrest now, and investigate later for probable cause." Id.