United States. Government Accountability Office. Nuclear Nonproliferation: Further Actions Needed by U.S. Agencies to Secure Vulnerable Nuclear and Radiological Materials, text, March 14, 2012; (https://digital.library.unt.edu/ark:/67531/metadc302888/m1/1/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .