In August 1848, the United States created the Oregon Territory out of territory gained with the settlement of the Oregon Question in 1846. Holbrook was appointed as the first United States Attorney for the territory by President Zachary Taylor, and arrived in Oregon in May 1850 with his family.Guide to the Amory Holbrook Papers, 1845-1861. Washington State University Libraries. Retrieved on March 3, 2008. In June, he served as the prosecution in the trial of the Native Americans charged with carrying out the Whitman Massacre.Carey, Charles Henry. (1922). History of Oregon. Pioneer Historical Publishing Co. p. 363. The five members of the Cayuse were convicted and hung in Oregon City.
