In 1967 Cook became active in a growing leftist paramilitary underground in Seattle, which perpetrated a series of high profile bombings and robberies. In and out of prison, he was co-founder of the Black Panther Party chapter in the Walla Walla State Penitentiary and served as its Lieutenant of Information for many years. In 2000, he was released after serving 24 years in prison for his participation in a bank robbery and jail break associated with the George Jackson Brigade in Seattle.