Customs and Border Protection - http://www.cbp.gov/
Customs and Border Protection is one of the Department of Homeland Security’s largest and most complex components, with a priority mission of keeping terrorists and their weapons out of the U.S. It also has a responsibility for securing and facilitating trade and travel while enforcing hundreds of U.S. regulations, including immigration and drug laws.
United States Secret Service - http://www.secretservice.gov
The United States Secret Service is a federal law enforcement agency with headquarters in Washington, D.C., and more than 150 offices throughout the United States and abroad. The Secret Service was established in 1865, solely to suppress the counterfeiting of U.S. currency. Today, the agency is mandated by Congress to carry out dual missions: protection of national and visiting foreign leaders, and criminal investigations.
Federal Bureau of Investigation - http://www.fbi.gov
As an intelligence-driven and a threat-focused national security and law enforcement organization, the mission of the FBI is to protect and defend the United States against terrorist and foreign intelligence threats, to uphold and enforce the criminal laws of the United States, and to provide leadership and criminal justice services to federal, state, municipal, and international agencies and partners.
Bureau of Alcohol, Tobacco, Firearms and Explosives - http://www.atf.gov/
A unique law enforcement agency in the United States Department of Justice that protects our communities from violent criminals, criminal organizations, the illegal use and trafficking of firearms, the illegal use and storage of explosives, acts of arson and bombings, acts of terrorism, and the illegal diversion of alcohol and tobacco products. We partner with communities, industries, law enforcement, and public safety agencies to safeguard the public we serve through information sharing, training, research, and use of technology.
United States Drug Enforcement Administration - http://www.justice.gov/dea/
The mission of the Drug Enforcement Administration (DEA) is to enforce the controlled substances laws and regulations of the United States and bring to the criminal and civil justice system of the United States, or any other competent jurisdiction, those organizations and principal members of organizations, involved in the growing, manufacture, or distribution of controlled substances appearing in or destined for illicit traffic in the United States; and to recommend and support non-enforcement programs aimed at reducing the availability of illicit controlled substances on the domestic and international markets.
United States Postal Inspection Service - https://postalinspectors.uspis.gov/
The mission of the U.S. Postal Inspection Service is to support and protect the U.S. Postal Service and its employees, infrastructure, and customers; enforce the laws that defend the nation’s mail system from illegal or dangerous use; and ensure public trust in the mail.
Disclaimer: This listing of Federal Law Enforcement Agencies lists only agencies charged with law enforcement as their primary day to day function. It is not a complete listing of all Federal law enforcement agencies as there are numerous Federal Agencies who, by Federal Law, employ Agents, Investigators and other Officers who posess Sworn Peace Officer Status in regards to investigating crimes and other matters related to that specific Agency.