The U.S. Supreme Court held in the case of Berghuis v. Thompkins, 560 U.S. 370 (2010), that a suspect in custody who has been read his Miranda rights must invoke his right to remain silent unambiguously. That is, if he does not expressly state that he wants to remain silent, the police need not end the interrogation, and need not ask for clarification of ambiguous conduct that may or may not indicate that the accused wants to remain silent.
On the other hand, if a suspect wants to waive his right to remain silent, he need not make a formal waiver. Ambiguous conduct is sufficient to waive Miranda rights. Thompkins, supra (citing North Carolina v. Butler, 441 U.S. 369, 376 (1979)).
Before a statement given after waiver of Miranda rights can be used against the suspect in court, the prosecution must show that the defendant voluntarily waived his rights, and that he knew what rights he was giving up. Although the Miranda decision in 1966 emphasized that the prosecution had a heavy burden to demonstrate a knowing and intelligent waiver, a later decision in the Miranda line of cases stated that the "heavy burden" is no more than a preponderance of the evidence. Thompkins, supra (citing Colorado v. Connelly, 479 U.S. 157, 168 (1986)).
In Thompkins, the suspect was taken into custody and shown a piece of paper that contained the Miranda warnings (right to remain silent; anything you say can and will be used against you in a court of law; you have the right to talk to a lawyer before answering and to have the lawyer present during questioning; if you cannot afford a lawyer one can be appointed to represent you before any questioning; you have the right to decide at any time before or during questioning to use your right to remain silent and to talk with a lawyer).
The Thompkins suspect was asked to read the last warning out loud to determine if he could read and understand English. The Court also noted that there was no basis in the case to conclude that the defendant did not understand his rights. Nor was there any contention by the defense that the suspect did not understand his rights.
The interrogation by police was skillfully done. The suspect was silent for most of the three hours of interrogation, with limited “yeah,” “no,” or “I don’t know” responses. Then the detective asked, “Do you believe in God?” The suspect then admitted that he did believe in God, as his eyes welled up with tears. The detective asked, “Do you pray to God?” The suspect answered “yes.” The detective asked, "Do you pray to God to forgive you for shooting that boy down?” The suspect answered “yes” and looked away. But he refused to make a written confession.
The Court held that a "suspect who has received and understood the Miranda warnings, and has not invoked his Miranda rights, waives the right to remain silent by making an uncoerced statement to the police." Evidently the Court believed that the suspect’s confession, as described above, was uncoerced. The Court also emphasized that the suspect could have invoked his Miranda rights at any time, even after he had begun speaking.