Interesting times over in the land of the free and home of the brave, as the US Supreme Court has held in a 5-4 decision that the right to silence must be unambiguously (and probably expressly) asserted.
The full text of Berghuis v Thompkins 560 US (including the dissenting judgment) can be found here.
In the US, questioning of a suspect must cease at the point where the suspect expresses a desire to invoke their rights under Miranda v Arizona 384 US 436. The Sixth Circuit Court had ruled that Thompkins’ unresponsiveness in the face of questioning over a prolonged period amounted to an implied invocation of his right to remain silent. The Supreme Court overruled the Sixth Circuit by the narrowest of margins.
Kennedy J (joined by Roberts CJ and Scalia, Thomas and Alito JJ) at 17:
In sum, 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 anuncoerced statement to the police. Thompkins did not invoke his right to remain silent and stop the questioning. Understanding his rights in full, he waived his right to remain silent by making a voluntary statement to the police. The police, moreover, were not required to obtain a waiver of Thompkins’s right to remain silent before interrogating him.