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SPRINGFIELD, Ill. (AP) - Historians believe they've unraveled the mystery of a cryptic note Abraham Lincoln penned that doesn't identify the recipient by name and has a section clipped out.

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. The use of that phrase gave researchers their best clue.

They ran it through a searchable database they are compiling 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, it turns out, were conspiring to keep tabs on a New York political figure.

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