 " [ A ] valid waiver will not be presumed simply from the silence of the accused after warnings are given or simply from the fact that a confession was in fact eventually obtained [ ... ] the fact of lengthy interrogation ... before a statement is made is strong evidence that the accused did not validly waive his rights . In these circumstances the fact that the individual eventually made a statement is consistent with the conclusion that the compelling influence of the interrogation finally forced him to do so . It is inconsistent with any notion of a voluntary relinquishment of the privilege . " 
