How long can police hold a witness?

Answer 1

You must be brought before a neutral magistrate, meaning a judge, within 24 hours of arrest. If that does not happen, the defendant may be entitled to some monitary award if the state misses the deadline by a few hours. The case will not be dismissed for this reason.

Answer 2

In the 1991 case of Riverside v. McLaughlin, the Supreme Court said holding suspects longer than two days without judicial review is unconstitutional. Justice Sandra Day O'Connor wrote ... in a 5-4 ruling, that a 48-hour rule would permit "a reasonable postponement of a probable-cause determination while the police cope with the everyday problems of processing suspects through an overburdened criminal justice system."

Answer 3

Generally, the police have no right to detain someone simply because they believe the person was a witness to something. They have to have a "reasonable suspicion" that the person was involved in a crime in order to detain them for a brief period, initially. In order to hold them longer (what constitutes "longer" varies based on the jurisdiction and circumstances), the police need a higher level of evidence known as "probable cause" and will also need to file charges against the person.

That being said, federal law, as well as many states have provisions for holding someone under a "material witness warrant." The police cannot do this unilaterally, though. They have to apply to a court and show that the person has important information, and that they would likely not show up in court (in simple terms).