Radioactive materials are used throughout the U.S. for a variety of purposes, including to help treat cancer, sterilize food and medical instruments. But, given the potential for them to be used to create a dirty bomb, which uses conventional explosives to spread radioactive material, the NRC regulates their sale and use.

The need to closely guard the distribution of radioactive substances is particularly acute now, according to the report. National Nuclear Security Administration officials at the Energy Department told GAO investigators that current assessments of the “threat environment show an increasing interest in using radioactive material for making a dirty bomb,” the report says.

The NRC requires a paper license to buy lower quantities of radioactive material. But GAO investigators found that the licenses can be altered and used to make illegal purchases of the materials.