Early one morning, 50 years ago today, while his wife, Mary, slept upstairs, Ernest Hemingway went into the vestibule of his Ketchum, Idaho, house, selected his favorite shotgun from the rack, inserted shells into its chambers and ended his life. There were many differing explanations at the time: that he had terminal cancer or money problems, that it was an accident, that he'd quarreled with Mary. None were true. As his friends knew, he'd been suffering from depression and paranoia for the last year of his life. This man, who had stood his ground against charging water buffaloes, who had flown missions over Germany, who had refused to accept the prevailing style of writing but, enduring rejection and poverty, had insisted on writing in his own unique way, this man, my deepest friend, was afraid - afraid that the F.B.I. was after him. Decades later, in response to a Freedom of Information petition, the F.B.I. released its Hemingway file. It revealed that beginning in the 1940s J. Edgar Hoover had placed Ernest under surveillance because he was suspicious of Ernest's activities in Cuba. Over the following years, agents filed reports on him and tapped his phones. In the years since, I have tried to reconcile Ernest's fear of the F.B.I., which I regretfully misjudged, with the reality of the F.B.I. file. I now believe he truly sensed the surveillance, and that it substantially contributed to his anguish and his suicide.