What is the primary law enforcement agency for the United States Postal Service?

The Postal Inspection Service is the primary law enforcement agency for the United States Postal Service. This specialized agency is responsible for protecting the mail and its resources, investigating crimes related to the postal system, and enforcing postal laws. It plays a unique role in ensuring the security and integrity of the mail, which can involve a wide range of activities from investigating mail theft and fraud to ensuring the safe transport of hazardous materials.

The other options represent agencies with broader or different mandates. The Federal Bureau of Investigation primarily addresses federal crimes, national security threats, and other serious offenses. The Department of Homeland Security focuses on protecting the United States from various threats, including terrorism and natural disasters, rather than the operational integrity of postal services. The U.S. Marshals Service is involved in apprehending fugitives and managing and transporting federal prisoners, which does not directly relate to the specific needs of postal law enforcement. Thus, the Postal Inspection Service is the designated authority for law enforcement within the context of the Postal Service's operations.