The arrest, and the discovery of the body, came yesterday after about 300 Spanish officers searched the area where Denise Thiem, 41, of Arizona was last seen April 5 in the town of Astorga.
Authorities were trying to determine whether the body was that of Thiem, said the security official, who spoke on condition of anonymity because of rules preventing the official from being named.
Astorga is on the El Camino de Santiago pilgrimage route, also called the St James Way, which ends at the Roman Catholic cathedral in the city of Santiago de Compostela.
Spanish TV showed footage yesterday of officers at a small apartment building taking away the suspect, a bearded man who appeared to be in his 30s. He was not identified.
The US Embassy in Madrid declined comment about the arrest of the suspect, citing privacy laws that prevent the disclosure of information about the case.
Thiem's disappearance prompted safety concerns about the 760-kilometer series of pilgrimage routes, which tens of thousands of people walk annually. Some do it as a spiritual quest, while others go for a physical challenge or hike it for a vacation.