From the government website:
On November 29, 1864, soldiers from the US military attacked a peaceful encampment of Cheyenne and Arapaho along Sand Creek. Over 150 Indians were killed in the attack, most of whom were women, children, or elderly. The location of the Sand Creek Massacre site was obscured through time even to descendents of massacre survivors. The Sand Creek Massacre National Historic Site Study Act of 1998 directed the National Park Service to identify the location the massacre area and evaluate the suitability designating the site as a national park unit.
Family stories from Cheyenne and Arapaho about the massacre were used to help identify the location of the massacre site. Tribes had the opportunity to conduct their own “oral histories.” The tribal investigations were conducted by descendents of massacre survivors and tribal leaders. Historians also searched archives for the story of Sand Creek in maps, diaries, testimonies from soldiers and Indians, newspaper articles, homestead records, military scouting reports, and historic photos.