There are a number of law enforcement agencies. At the federal level, there are a large number of law enforcement agencies and their jurisdiction decides their operation.
Let's discuss Department of Energy (DoE), US Customs and Border Protection (CBP) and Department of Defense (DoD). The DoE is concerned primarily with energy, including energy production, nuclear material handling and disposal, nuclear programs, etc. Apart from that, the DoE also carries out extensive research through a series of national laboratories. The CBP is concerned with regulations regarding international trade, immigration, customs, etc. The primary responsibility of the CBP is to ensure that US borders are protected from the entry of terrorists and people with criminal records/intentions, drugs/contraband and also to protect agriculture from pests and insecticides. The DoD deals mainly with national security and armed forces, and is the largest employer in the world with over 3.2 million employees. Each agency has a different organizational hierarchy (depending on the various departments operating under it), enforcement powers and programs that it can undertake.