“ In 1964 petitioner was tried and convicted in a Massachusetts Superior Court for murder, armed robbery, and other offenses. The conviction was affirmed by the Supreme Judicial Court of Massachusetts. Commonwealth v. Johnson, 352 Mass. 311, 225 N.E.2d 360. We granted certiorari because there appeared to be substantial questions concerning the voluntariness of a confession of petitioner which was admitted in evidence at his trial. ”