Jeff Campbell worked for 20 years as a criminal investigator for the state of New Mexico. He specialized in cold cases. These days, he applies his sleuthing skills to a case so cold it’s buried beneath a century and a half of windblown prairie.

“Here’s the crime scene,” Campbell says, surveying a creek bed and miles of empty grassland. A lanky, deliberate detective, he cups a corncob pipe to light it in the flurrying snow before continuing. “The attack began in predawn light, but sound carries in this environment. So the victims would have heard the hooves pounding towards them before they could see what was coming.”

Campbell is reconstructing a mass murder that occurred in 1864, along Sand Creek, an intermittent stream in eastern Colorado. Today, less than one person per square mile inhabits this arid region. But in late autumn of 1864, about 1,000 Cheyenne and Arapaho lived in tepees here, at the edge of what was then reservation land. Their chiefs had recently sought peace in talks with white officials and believed they would be unmolested at their isolated camp.

When hundreds of blue-clad cavalrymen suddenly appeared at dawn on November 29, a Cheyenne chief raised the Stars and Stripes above his lodge. Others in the village waved white flags. The troops replied by opening fire with carbines and cannon, killing at least 150 Indians, most of them women, children and the elderly. Before departing, the troops burned the village and mutilated the dead, carrying off body parts as trophies.

There were many such atrocities in the American West. But the slaughter at Sand Creek stands out because of the impact it had at the time and the way it has been remembered. Or rather, lost and then rediscovered. Sand Creek was the My Lai of its day, a war crime exposed by soldiers and condemned by the U.S. government. It fueled decades of war on the Great Plains. And yet, over time, the massacre receded from white memory, to the point where even locals were unaware of what had happened in their own backyard.

That’s now changed, with the opening of the Sand Creek Massacre National Historic Site. “We’re the only unit in the National Park Service that has ‘massacre’ in its name,” says the site’s superintendent, Alexa Roberts. Usually, she notes, signs for national historic sites lead to a presidential birthplace or patriotic monument. “So a lot of people are startled by what they find here.”