Final answer:
The United States Postal Inspection Service investigates various crimes, including fraud, credit card fraud, identity theft perpetrated through the mail, and mailing of illegal substances.
Step-by-step explanation:
The United States Postal Inspection Service investigates cases of fraud, credit card fraud, identity theft perpetrated through the mail, and interstate mailing of illegal substances using the mail system. In order to combat certain threats such as bioterrorism, both the USPS and the FBI have taken measures such as installing x-ray generators to irradiate suspicious looking mail, which serves to eliminate most bacteria and some viruses. These x-rays can alter the physical properties of the mail, but do not make it radioactive.