SPRINGFIELD, Ill. - The cryptic note penned by Abraham Lincoln identifies its recipient only as "my dear Sir" and has a small section carefully clipped out.
Who was he writing to and why was a key piece of information later removed so meticulously?
Historians believe they have unraveled the mystery and uncovered a bit of political intrigue in the process.
Researchers at the Papers of Abraham Lincoln project concluded Lincoln was writing to an ally to ask him to maintain a secret relationship with a political insider during the 1860 election campaign.
Lincoln asked his cohort to "keep up a correspondence" with the person, a phrase that gave researchers their best clue. They ran it through a searchable database of Lincoln's papers and found several matches.
One was in a letter to Lincoln from fellow attorney and Republican Leonard Swett of Bloomington, Ill.
The two men, it turns out, were conspiring to keep tabs on a New York political figure. The mystery note was Lincoln's response to Swett's letter, the researchers surmised.
"If you can keep up a correspondence with him without much effort, it will be well enough," Lincoln wrote to Swett. "I like to know his views occasionally."
Swett's earlier letter also had a clue about who the political insider was. It referred to "our friend TW of Albany," who researchers concluded was Thurlow Weed, a Republican newspaper editor and political boss of New York state.
Lincoln was seeking Weed's support in New York, even though Weed had been backing front-runner William H. Seward for the Republican presidential nomination. Lincoln got his way, ultimately winning Weed's support. Seward later became his secretary of state.