In a memoir being published posthumously, Senator Edward M. Kennedy talks remorsefully about the car accident that claimed the life of Mary Jo Kopechne – a turn of events many consider a chief reason that he was never able to mount a successful bid for the presidency.

Writing in his book “True Compass,” which is scheduled to be published on Sept. 14, Mr. Kennedy, who died a week ago, described his actions in the 1969 accident as “inexcusable” and said that at the time he was afraid, overwhelmed “and made terrible decisions.”

Mr. Kennedy said he had to live with the guilt of his actions for four decades but that Ms. Kopechne’s family had to endure worse. “Atonement is a process that never ends,” he writes.

In the 532-page book, a copy of which was obtained by The New York Times, Mr. Kennedy also said he has always accepted the official findings of President John F. Kennedy’s assassination, an event that he said left family members fearing for the emotional health of his brother, Robert F. Kennedy. Mr. Kennedy that he often thought of one brother’s deep grief over the loss of another and said it “veered close to being a tragedy within a tragedy.”