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Mass violence has "a long, ugly history" in America, said Katherine Grandjean, history professor at Wellesley College. "It's a pattern as old as the nation and goes back long before even into the colonial roots."

The victims have often been racial, ethnic or religious minorities. The perpetrators have ranged from military troops to mobs to a single shooter.

Some examples:

— In 1857, 120 people traveling on a wagon train heading to California were shot and killed by Mormons as they went through Utah. It was known as the Mountain Meadows Massacre.

— In 1864, Colorado volunteer soldiers attacked an encampment of Cheyenne and Arapaho Indians in the Sand Creek Massacre, killing at least 200 people. Victims' bodies were mutilated.

— In 1890, 300 Indian men, women and children were shot to death by the 7th Cavalry on the Pine Ridge reservation in South Dakota in what came to be known as the Wounded Knee Massacre.