(CNN) -- The devices used in the Boston Marathon attack Monday are typical of the "lone wolf:" the solo terrorist who builds a bomb on his own by following a widely available formula. In this case, the formula seems very similar to one that al Qaeda has recommended to its supporters around the world as both crudely effective and difficult to trace. But it is also a recipe that has been adopted by extreme right-wing individuals in the United States. The threat of the "lone wolf" alarms the intelligence community.