Shock Corridor (1963)

Johnny Barrett, an ambitious journalist, is determined to win a Pulitzer Prize by solving a murder committed in a lunatic asylum and witnessed only by three inmates, from whom the police have been unable to extract the information. With the connivance of a psychiatrist, and the reluctant help of his girlfriend, he succeeds in having himself declared insane and sent to the asylum. There he slowly tracks down and interviews the witnesses - but things are stranger than they seem ...—David Levene