377 U.S. 201 (1964), argued 3 Mar. 1964, decided 18 May 1964 by vote of 6 to 3; Stewart for the Court, White in dissent. Massiah was decided at a time when the Warren Court's “revolution in American criminal procedure” was accelerating. According to Massiah, after the initiation of adversary judicial proceedings (by indictment, as in Massiah's case, or by information, preliminary hearing or arraignment), the Sixth Amendment guarantees a defendant the right to rely on counsel as the “medium” between himself and the government. Thus, once adversary proceedings have begun, the government cannot bypass the defendant's lawyer and deliberately elicit statements from the defendant himself.
Kamisar, Yale. "Massiah v. United States." In The Oxford Guide to United States Supreme Court Decisions. 2d ed., edited by K. L. Hall and J. W. Ely Jr., 208-10. Oxford: Oxford Univ. Press, 2009.