From the Bureau of Justice Statistics
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.