After Booth assassinated Lincoln on April 14, 1865, Arnold was arrested from his job in Virginia on suspicion of complicity. He admitted his part in the plot to kidnap Lincoln, and his co-workers supported his contention of being in Virginia at the time of the assassination. During the trial, one of the chief witnesses was Louis J. Weichmann, a boarder at Mary Surratt's (John Surratt's mother).