An ailing Mark Twain pleaded with his business manager to kill him aboard a steamship as he traveled home to die, a long-lost letter shows.

In intense pain from a chronic heart ailment, he asked his agent, Albert Bigelow Paine, to end it all on the high seas, the letter says.Paine traveled with Twain on the S.S. Oceana to New York from Twain's holiday home in Bermuda in April 1910 and described the incident in a letter to U.S. diplomat William Allen. Twain died later that month.

"From the time we left until we ran out of the Gulf Stream air, he was obliged to sit or stand straight up constantly and we expected him to die any moment," wrote Paine. "We never left him alone.

"At times his mind wandered and his struggles for breath were terrible to see. He begged me to kill him."

The Oceana docked in New York on April 14 and Paine traveled with the author to his Stormfield estate in West Redding, Conn., where he died on April 21, 1910.