The sixth amendment guarantees to the accused in a criminal prosecution the right "to have the Assistance of Counsel for his defence." In Massiah v. United States, the Supreme Court held this right was violated when there was used against the defendant at trial evidence of incriminating statements deliberately elicited from him by an informant after he had been indicted and in the absence of counsel. In effect, this decision and others that 'followed have created a new constitutional right not to be questioned about pending charges prior to trial except in the presence of an attorney.
One consequence of the Massiah line of cases is that federal and state investigators may not use otherwise legitimate investigative methods to obtain incriminating statements from a person once he has been charged with a crime. Another is that federal and state prosecutors may not introduce at trial probative, reliable, and voluntary admissions of guilt by the defendant. The overall result is that the search for truth in criminal investigations and trials is thwarted.
Department of Justice Office of Legal Policy,
The Sixth Amendment Right to Counsel under the Massiah Line of Cases,
U. Mich. J. L. Reform
Available at: https://repository.law.umich.edu/mjlr/vol22/iss3/7