Mark Greaney has a degree in international relations and political science. In his research for the Gray Man novels, including Sierra Six, Relentless, One Minute Out, Mission Critical, Agent in Place, Gunmetal Gray, Back Blast, Dead Eye, Ballistic, On Target, and The Gray Man, he traveled to more than thirty-five countries and trained alongside military and law enforcement in the use of firearms, battlefield medicine, and close-range combative tactics. With Marine LtCol Rip Rawlings, he wrote the New York Times bestseller, Red Metal. He is also the author of the New York Times bestsellers Tom Clancy Support and Defend, Tom Clancy Full Force and Effect, Tom Clancy Commander in Chief, and Tom Clancy True Faith and Allegiance. With Tom Clancy, he coauthored Locked On, Threat Vector, and Command Authority.