In the case of Kirby v. Illinois, the U.S. Supreme Court ruled that an individual's Sixth Amendment right to counsel does not apply until a prosecution is formally initiated, which typically occurs at or after indictment or arraignment. The decision came in response to a challenge by two men who were arrested and identified in a police lineup without having legal representation present. They argued this violated their constitutional rights under the Sixth Amendment, which guarantees defendants access to legal counsel during "critical stages" of criminal proceedings against them. However, the court disagreed with this interpretation and held that pre-indictment lineups do not constitute such critical stages because they are preliminary investigative procedures rather than parts of formal prosecutions.

In the dissenting opinion for Kirby v. Illinois, Justice Brennan disagreed with the majority's decision that a suspect’s Sixth Amendment right to counsel does not apply before formal charges are filed. He argued that this interpretation was too narrow and failed to adequately protect individuals' constitutional rights during critical stages of criminal proceedings. Brennan contended that any confrontation between an accused and law enforcement should trigger the right to legal representation, regardless of whether formal charges have been brought or not. This would ensure fair treatment and prevent potential abuses by police during pre-charge investigations or interrogations. The justice also expressed concern about limiting access to counsel based on procedural technicalities rather than focusing on substantive fairness in criminal prosecutions.