And here's also an official National Park narrative:

Big Foot's people camped to the south of the soldiers along the north edge of a dry ravine. Little movement occurred in the Indian camp that evening, although Dewey Beard, a young man at the time and one of the Lakota survivors, later said, "There was a great uneasiness among the Indians all night [for fear] that they were to be killed."

December 29, 1890

On the morning of December 29 Colonel Forsyth called a council. He ordered the Lakotas to surrender all of their guns and told them they would be taken to another camp. This immediately started a rumor among the Indians that they were to be taken to Indian Territory in what is now Oklahoma - a fate worse than prison to the Minneconjous. The Lakotas grudgingly surrendered a few old weapons, but because Forsyth believed the Indians were hiding their best rifles, he ordered a search of the warriors and the camp.

While a few soldiers rummaged through the Indians' tents for weapons, an Indian from the council circle began singing Ghost Dance songs and "stooping down, took some dirt and rose up facing the west ... cast the dirt with a circular motion of his hand toward the soldiers." Lt. John C. Gresham later said that when he saw this action, he and the rest of the soldiers interpreted it to be a signal to attack the troops. But Dewey Beard later explained that the man threw the dirt "as they did in the ghost dance when they call for the Messiah."