There is a long-standing debate about the authorship of the Bixby Letter, one of the most famous pieces of correspondence in American history. Despite being signed by President Abraham Lincoln, some historians have claimed that its true author was John Hay, Lincoln’s personal secretary. Analyses of the letter have been inconclusive in part because the text totals only 139 words and is thus far too short to be attributed using standard methods. To test whether Lincoln or Hay wrote this letter, we therefore introduce and apply a new technique for attributing short texts called ‘n-gram tracing’. After demonstrating that our method can distinguish between the known writings of Lincoln and Hay with a very high degree of accuracy, we use it to attribute the Bixby Letter. We conclude that the text was authored by John Hay—rewriting this one episode in the history of the USA, while offering a solution to one of the most persistent problems in authorship attribution.