The incidents could fall into five different categories: airborne clutter, natural atmospheric phenomena, classified US programmes, foreign adversary systems from countries such as China and Russia, or other. The report which was sent to US Congressional committees said the "limited amount of high-quality reporting" on the phenomena "hampers our ability to draw firm conclusions." UFOs have long been part of science fiction folklore but recently officials have highlighted the national security concerns surrounding the unidentified phenomena, particularly near defence sites. "It is critical that the United States maintain operations security and safety at DoD ranges," US deputy secretary of defence Kathleen Hicks said in a memo released after the report.