The Sand Creek Massacre Entry from the "Timeline of American-Indian Relations" in Fleming & Luskey, The North American Indians in Early Photographs:

1864, November, Sand Creek massacre, Colorado. A camp of six hundred Cheyenne and Arapaho is attacked by Colonel John M. Chivington with a force of Colorado militia, despite of assurances of safety from Major Scott C. Anthony and Major Edward W. Wynkoop. 105 women and children are killed, with 28 men. The effect of the massacre is to destroy the position of chiefs such as Black Kettle who wanted peace with the United States; it also paves the way for a treaty the following year which effectively means the abandonment of all Cheyenne and Arapaho claims to Colorado Territory.

Eye witness report of Robert Bent quoted in Brown (89):

I saw the American flag waving and heard Black Kettle tell the Indians to stand around the flag, and there they were huddled  men, women, and children. This was when we were within fifty yards of the Indians. I also saw a white flag raised. These flags were in so conspicuous a position that they must have been seen. When the troops fired, the Indians ran, some of the men into their lodges, probably to get their arms. ... I think there were six hundred Indians in all. I think there were thirty-five braves and some old men, about sixty in all ... the rest of the men were away from camp, hunting ...
 

A soldier's eye witness report quoted in Brown (90):

In going over the battleground the next day I did not see a body of man, woman, or child but was scalped, and in many instances their bodies were mutilated in the most horrible manner - men, women and children's privates cut out, &c; I heard one man say that he had cut out a woman's private parts and had them for exhibition on a stick; I heard another man say that he had cut the fingers off an Indian to get the rings on the hand; according to the best of my knowledge and belief these atrocities that were committed were with the knowledge of J.M. Chivington, and I do not know of his taking any measures to prevent them; I heard of one instance of a child a few months old being thrown in the feed-box of a wagon, and after being carried some distance left on the ground to perish; I also heard of numerous instances in which men had cut out the private parts of females and stretched them over the saddle-bows and wore them over their hats while riding in the ranks.