Current and former staff of American not-for-profits with experience in North Korea have expressed doubts about key claims related to a "missionary spies" intelligence-gathering program that surfaced on Monday.

The program, said to have existed from 2004 to 2013, was claimed to have used a now defunct U.S. NGO to smuggle "scores of bibles" into North Korea in order to "test run" a smuggling route on behalf of the Pentagon, online outlet the Intercept reported on Monday.