In a court filing in early March, lawyers for the committee investigating the Jan. 6, 2021, Capitol insurrection charged that former president Donald Trump and key allies engaged in two potential crimes: conspiring to defraud the United States and obstructing an official congressional proceeding. The claim is the latest chapter in the extensive — and ongoing — congressional investigation into the insurrection and offers some early clues as to the committee’s thinking and what its eventual report will look like.

If history is any guide, that report will be an incredibly important document. After the Civil War, defeated ex-Confederates turned to terrorism to limit the possibilities of freedom for formerly enslaved people. The violence only grew after Black men won the right to vote. Congressional Republicans — members of the party of Abraham Lincoln, emancipation and Black men’s enfranchisement — responded by forming a joint House-Senate committee to investigate “the execution of the laws, and the safety of the lives and property of the citizens of the United States” in the former Confederate states. Its report, submitted 150 years ago, holds sobering lessons for the Jan. 6 committee about what comes next: that is, how such an inquiry can succeed or fail in influencing policy and in shaping later understandings of a perilous chapter in the nation’s history.

Republicans outnumbered Democrats 13 to eight on the Joint Select Committee to Inquire Into the Condition of Affairs in the Late Insurrectionary States — popularly known as the Ku Klux Committee, named after the secret terrorist organization responsible for much of the violence. The Democratic minority, however, included racist and outspoken opponents of Reconstruction. One was Sen. Francis Blair Jr. (Mo.), who had once denounced African Americans as an “alien race of semi-barbarous men.” Republicans did their best to uncover the Klan’s goals and the extent of its terrorism, but Democrats on the committee rejected the proceedings from the outset, creating a partisan spectacle. They worked to put biracial democracy itself on trial, badgering and belittling Black witnesses and calling White witnesses to complain of the incompetence and venality of officials whom Black voters helped elect.

The committee’s reports and supporting evidence totaled 13 volumes and more than 8,000 pages, at the time the longest published congressional investigation in history. It contained the testimony from 600 witnesses, about one-third of them African Americans, who told of violent assaults and murders, rapes and arson committed by the Klan. The report from the Republican majority highlighted harrowing stories of political terror, including the account of Elias Hill, a disabled teacher, Baptist minister and grass-roots political leader who was brutally beaten outside his South Carolina home by six Klansmen.

The Democratic minority, however, submitted its own report shot through with racist and inflammatory language. It denounced “Negro supremacy” and called the enfranchisement of Black men “one of the most terrible blunders ever committed.” Democrats minimized the Klan’s “alleged outrages,” claiming that the Klan was not a political force and thus could not be responsible for political terrorism, but simultaneously excusing Klan violence as a just political response — “the legitimate offspring” — of Republican power in the South.

In the short term, little came of the investigation. The Republican majority recommended extending an existing law that allowed the president to suspend the writ of habeas corpus to effect mass arrests. President Ulysses S. Grant had put it to good use a few months earlier, employing the U.S. Army to help quash terrorism in South Carolina. But with a presidential election approaching, congressional Republicans chose instead to moderate their approach on the “Southern question” and let that provision lapse. It was a step in the national Republican Party’s retreat from Reconstruction and weakened the federal government’s ability to respond to future violence.