"In September 2008, federal agencies employed approximately 120,000 full-time law enforcement officers who were authorized to make arrests and carry firearms in the United States. This was the equivalent of 40 officers per 100,000 residents. The number of federal officers in the United States increased by about 15,000, or 14%, between 2004 and 2008. Federal agencies also employed nearly 1,600 officers in the U.S. territories in 2008, primarily in Puerto Rico. These findings come from the 2008 Census of Federal Law Enforcement Officers (FLEO), conducted by the Bureau of Justice Statistics (BJS). The 2008 FLEO census collected data from 73 agencies, including 33 offices of inspectors general. The largest number of federal officers in the United States (about 45,000, or 37%) performed criminal investigation and enforcement duties (figure 1). The next largest job function category was police response and patrol with about 28,000 officers (23%). Approximately 18,000 officers (15%) performed immigration or customs inspections, and about 17,000 (14%) performed corrections or detention-related duties. Other federal officers primarily performed duties related to security and protection (5%) or court operations (5%). Excluding offices of inspectors general, 24 federal agencies employed 250 or more full-time personnel with arrest and firearm authority in the United States (table 1). These agencies employed 96% of all federal officers. The four largest agencies, two in the Department of Homeland Security (DHS) and two in the Department of Justice (DOJ), employed about two-thirds of all officers. Overall, DHS and DOJ agencies employed about 4 in 5 federal officers."
United States. Bureau of Justice Statistics: http://www.bjs.gov/