Gregory Ulas Powell (August 2, 1933 – August 12, 2012) was an American criminal who kidnapped Los Angeles Police Department Officers Ian Campbell and Karl Hettinger on the night of March 9, 1963. Assisted by accomplice Jimmy Lee "Youngblood" Smith, Powell took the officers to an onion field near Bakersfield, California, where Officer Campbell was fatally shot. Infamously known as the "Onion Field" Killer, Powell's story was depicted in Joseph Wambaugh's 1973 non-fiction book, The Onion Field. The book was later made into a 1979 film adaptation of the same name in which Powell was portrayed by James Woods.
Powell was raised in Michigan by a dysfunctional family. His father was a musician who was often absent during his childhood, leaving Powell to take care of his three younger siblings. At age 15, Powell ran away from his home and hitchhiked to Florida, where he met a Catholic priest and together they had a brief sexual relationship. At age 16, he stole money and a car from his sister; he stole another car when he was 18. Powell served time in Michigan for car theft. When he was 20, Powell was released from prison. At one point in his life, after he and his family moved to California, Powell underwent either a craniectomy or a craniotomy at Vacaville Medical Facility to remove a brain tumor. Powell was frequently in and out of prison; by the time he was 29 years old, ten of the last thirteen years of his life were spent behind bars. He was officially paroled in May 1962. Powell worked as thief and had often robbed at liquor stores. He also worked as a gay hustler. As of January 1963, Powell had been responsible for a series of armed robberies in Las Vegas. At the time of the kidnapping, Powell was residing in Boulder City, Nevada.