The decision was revealed in a recent Department of Energy letter to the Federation of American Scientists, a private group that studies nuclear weapons issues and advocates for government transparency on national security issues.

"After careful consideration ... it was determined that the requested information cannot be declassified at this time," the Energy Department wrote in an April 5 letter responding to the federation's request. The Department provided no explanation for the decision, which it said was made by the Formerly Restricted Data Declassification Working Group, consisting of officials from the Departments of Defense and Energy.