"Government officials have struggled to strike the right balance between security and freedom at historical sites in the United States, with some monuments or national parks featuring stringent airport like security measures while others have a minimal security presence; the General Accounting Office (GAO) has issued two reports, one in 2003 and one in 2009, that found security at U.S. monuments were in need of improvement; The National Park Service (NPS) has more than doubled the size of its security staff, increasing its numbers from 1,500 to nearly 4,000 following the 9/11 attacks; so far, there have been no credible threats to monuments in the United States, but since the five most secure U.S. monuments have a single access entry, they are potential targets for a mass casualty attack."