A mystery man — who wrote congressional legislation, rode with and influenced Maj. Gen. George B. McClellan and knew Washington well — has surfaced as the author of the District of Columbia Compensated Emancipation Act.

Col. Thomas M. Key, a master at behind-the-scenes politics, allowed Sen. Henry Wilson of Massachusetts to claim responsibility for writing the measure that would immediately free some 3,000 slaves living in the District but also compensate their loyal Union owners with up to $300 per slave.

On April 16, 1862, President Abraham Lincoln signed the legislation into law, setting off joyous street celebrations. Each year thereafter, until 1901, the black residents of the District marked the anniversary with a huge parade. The annual celebration was revived in the 1990s and is now recognized as an official holiday in the District. Local government offices will be closed and the city has announced several events for Wednesday, including a parade and concert.

According to the 2012 book, “McClellan’s Other Story: The Political Intrigue of Colonel Thomas M. Key, Confidential Aide to General George M. McClellan,” by historian William Styple, Key was a well-connected Cincinnati lawyer, former judge and state senator. He arranged to have Wilson introduce the emancipation bill in December 1861. Key and Wilson knew each other from a short stint the senator spent on McClellan’s staff.

Popularly known as Wilson’s emancipation bill, it was fiercely debated with abolitionists lining up against pro-slavery factions in Congress. When it finally passed and Lincoln signed it into law, Key was delighted because it was very close to what he had written. Wilson was also delighted because he could take credit for writing a bill that made him an instant hero to abolitionists everywhere.

Styple’s meticulous research over two decades has documented the war-era activities of Key, who held the position of judge advocate on McClellan’s staff. He was also referred to as the general’s political adviser (McClellan would run for the presidency in 1864, winning the Democratic nomination). The two men were close friends before the war started, having met in 1860, when McClellan moved to Cincinnati, after resigning from the Army.

However, when it came to war-time activities, Key was an enigma. His name does not appear in McClellan’s official papers, he avoided group photographs with McClellan and he instructed his lawyer to destroy all his personal papers when he died.

Styple was able to gather information from other sources. He found newspaper stories about Key and an obituary that tied him to the D.C. Emancipation Act. He also searched through forgotten files, private correspondence, published and unpublished memoirs.

Styple said in a telephone interview that no one knows why Key was so secretive during the war, but it may be tied to his belief that by resolving the slavery issue, he could end the war. “I believe Key, and also McClellan, wished to end the rebellion with little bloodshed,” he said. “The District Emancipation Bill was meant to serve as a template, a law written to show the Southern slave holders that the federal government would compensate them for their loss of property.”