Mystery of cryptic Abraham Lincoln letter addressed ‘My dear Sir’ finally solved

The secret behind a mysterious letter sent by Abraham Lincoln to an associate named only as “my dear Sir” has finally been unravelled by a team of dedicated experts, more than 150 years later.

The future president wrote the letter to an ally during the 1860 election campaign, asking him to maintain a discreet relationship with a political insider – whose name has been deliberately removed.

In full – which isn’t saying much – the letter reads: “Mr dear Sir, I thank you for the copy of [clipped section]. If you can keep up a correspondence with him without much effort, it will be well enough. I like to know his views occasionally. Yours in haste. A Lincoln.”

Researchers at the Papers of Abraham Lincoln project decided that the unusual phrase “keep up a correspondence” might be 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, Illinois. 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.

Swett's earlier letter also had a clue about who the political insider was. It referred to “our friend TW of Albany,” who researchers deduced 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.

Swett was recruited to be a secret go-between and the letter was later altered to remove any tell-tale details because, researchers think, Lincoln couldn't be seen as close to Weed during the campaign.

A New York City manuscript dealer recently contacted the Papers of Abraham Lincoln group for help solving the riddle.

The team of researchers is trying to identify, transcribe and publish all documents written by or to Lincoln. Project Director Daniel Stowell said Saturday that solving the mystery behind the note points to the project's value.

“To be able to identify the date, recipient and subject of such a brief letter is a remarkable achievement,” he said.

Additional reporting by AP