Bureau 13 (the 13th Bureau of the Justice Department) was founded in 1862 by Abraham Lincoln to deal with supernatural and paranormal threats to the Union (and suppress any public knowledge of them). The original agents were a motley crew of military personnel, Pinkerton detectives, civilian consultants, freed slaves, paroled criminals, and even Confederate prisoners of war.
After the war, The Bureau had access to a large budget and limitless resources. Safe houses and caches of equipment were stockpiled across the country. The headquarters in Washington, D.C. later became the East Coast office. A West Coast office was created in Silver City, New Mexico Territory to speed up response times to threats on the frontier. After the Indian Wars and the settling of the West, Bureau 13 found itself involved in threats outside the United States, providing its services during World War I, World War II, and the Cold War.
On 7 July 1977, a highly coordinated attack destroyed the main offices and killed hundreds of agents in what was later dubbed the "'77 Massacre". The organization splintered into groups of survivors that took years to reform. The remaining agents decided to go underground as a cell-based network. Gradually they began to create networks of "friendlies" - assets and sympathizers that would help the Bureau when called upon. As the number of original agents retired or died, the "friendlies" started to be promoted to agent status. Most agents encountered are usually civilians who received their training on the job.
The job of Bureau 13 is as it has always been. Investigate the strange or unusual, analyze the evidence to see if there is a supernatural or paranormal cause, and assess whether the cause is hostile or dangerous.
Source: Wikipedia, "Bureau_13 Background", available under the CC-BY-SA License.