A stretch of dry, empty prairie where the Sand Creek Massacre took place in Colorado has hardly changed in a century and a half.

Back in December 1864, America was still months from the end of the Civil War. Gen. William Sherman was making his infamous march across Georgia. And from the Western Frontier, word of the shocking Sand Creek Massacre was starting to trickle out. A regiment of volunteer troops in Colorado had attacked a peaceful camp of Native Americans, slaughtering nearly 200 of them — mostly women and children.

The creek itself is just a curve of sand and a scattering of cottonwoods. But for Karen Little Coyote of the Cheyenne tribe, this is a sacred place.

"Something comes over me each time I come out here. You can feel the spirits out here," Little Coyote says.

In 1864, Cheyenne and Arapaho chiefs brought their people to Sand Creek to wait for peace negotiations with the territorial government. Instead, the village was attacked early one morning by United States cavalry, a volunteer regiment led by a colonel bent on driving Indians out of the territory. Little Coyote's great-great grandfather Chief Black Kettle survived the massacre.

"You can stand there and you can just imagine what happened out here. Women, children, screaming and crying and don't know what's going on," Little Coyote says.

Hundreds of Cheyenne and Arapaho returned to the massacre site recently to mark the 150th anniversary with private ceremonies and a public education program. Martin Braided Hair stretched white canvas over a frame of poles, one of several teepees the tribes put up for their rituals. Braided Hair says visiting the massacre site is an important way for the Cheyenne to connect with their history — from their near-annihilation at Sand Creek to their struggles to survive today.

"We're having a hard time with our language, with our way of life. And each time we come back, it gets stronger and stronger," he said.

Congress made this stretch of Sand Creek a National Historic Site less than a decade ago and gave it a heavy mission. Alexa Roberts, superintendent of the site, says the goal is not just to remember the massacre but to use that memory to try to prevent future atrocities.

"This wasn't just an event in history; it wasn't something that just happened and is over," Roberts says. "The things that could bring about an atrocity of this magnitude — those were human things, and that potential is still in people.

The horror of Sand Creek didn't end with the massacre. Soldiers took scalps and other grisly trophies from the dead and brought them back to Denver for public display. The outrage of those actions prompted the tribes to start an annual healing run, leaving the site after the anniversary and following the same route the soldiers took back to Denver. Wilma Blackbear and Janet Bull Coming traveled from Oklahoma to take part this year.

"It's to heal ourselves ... pray, and give us strength to help move on," they said.

On a chilly Denver morning, a traditional chant followed tribe members and supporters as they set off on the final leg of the run. Their route ended at the state capitol, where Colorado's governor, John Hickenlooper, was waiting to do something none of his predecessors have done before: formally apologize for the Sand Creek Massacre.