The decision which determined that the peple had to be informed of their rights before being questions was the Miranda Rights. (Miranda vs. Arizona court case).
Miranda rights constitute the rights granted to people in the United States under arrest. These rights stem from a 1966 Supreme Court case known as Miranda v. Arizona.
In the case, the defendant called Ernesto Miranda, was a 24-year-old high school drop-out who had a police record at the time he was accused in 1963 of kidnapping, robbing and raping an 18-year-old woman. During a two-hour interrogation, Miranda confessed to the crimes.