s dawn broke over the eastern Colorado prairie on Nov. 29, 1864, a hastily assembled regiment of volunteer U.S. cavalrymen approached their target: a peaceful village of Cheyenne and Arapaho wintering on Sand Creek.

Somewhere in the ranks rode my great-great-grandfather William M. Allen.

His commander, a fiery former Methodist preacher, reminded the men of previous Indian attacks against settlers. “Now boys,” he thundered, “I shan’t say who you shall kill, but remember our murdered women and children.”

Over the next nine hours, the troopers slaughtered up to 200 people, at least two-thirds of them noncombatants, then mutilated the dead in unspeakable fashion. The Sand Creek Massacre scandalized a nation still fighting the Civil War and planted seeds of distrust and sorrow among Native Americans that endure to this day.

William M. Allen, who was 27 at the time, had enlisted in the Third Colorado Cavalry Regiment in response to an urgent call by the governor for volunteers to pursue “hostile Indians.” The ill-trained and poorly equipped unit, composed of farmers, miners, shopkeepers and tradesmen, served for just over 100 days, then melted back into civilian life. Allen went on to local prominence, building up substantial landholdings and serving as a county commissioner. A neighborhood and street in the Denver suburb of Arvada took his name. He rarely spoke of Sand Creek.

Now, as the 150th anniversary approaches, Native Americans are trying to restart the conversation. A commemoration will be held on the steps of the state Capitol in Denver. The United Methodist Church is investigating its culpability in the affair, given that key figures, including the commander, Col. John M. Chivington, were prominent members. Northwestern University and the University of Denver, both founded by territorial Gov. John Evans, have pored through thousands of pages of official records, letters and other documents. They want to reconcile how a man known for his Christian generosity could have been a party to such an atrocity—and whether they should feel any guilt by association.