Founded over 100 years before the FBI, the U.S. Postal Inspection Services is one of the oldest, and perhaps the least well known, of the law enforcement services in the US. Tasked with securing the >149 billion pieces of mail shipped annually through the U.S. Postal Service, its network of some 2,500 inspectors, police officers, technical specialists and administrators enforce laws pertaining to crimes carried out through the postal system. This has included the delivery of pipe bombs and other suspicious parcel response. In 2017, the agency made over 5,000 arrests and achieved over 4,000 convictions, mostly for mail theft and mailed narcotics. The service’s website indicates that their most important responsibility is the protection of postal employees, who often face violence and other risks on the job. Due to their smaller size, their investigations typically work in coordination with other federal agencies or local law enforcement.
Source: Delivering Security: Inside the law enforcement agency older than the US government