A more thorough explanation:

The United States Attorney’s Office (USAO) is a part of the Department of Justice that represents the federal government in court. The USAO is led by a United States Attorney (USA) who is appointed by the President and confirmed by the Senate. Assistant United States Attorneys (AUSA) work under the USA and handle most of the USAO’s legal cases.

The USAO has three main responsibilities:

1. Prosecute federal crimes
2. Represent the United States in civil actions
3. Collect debts owed to the federal government that are administratively uncollectible

The majority of the USAO’s work involves prosecuting federal crimes, such as:

• Drug trafficking
• Sex trafficking
• Child pornography
• Violations of federal firearm laws
• White-collar crime

The USA and AUSAs frequently appear in federal district courts and federal courts of appeal. They also work closely with Department of Justice investigative bodies, such as the FBI and the ATF, to develop cases.

For example, if someone is caught trafficking drugs across state lines, the USAO would be responsible for prosecuting them in federal court. The AUSAs would work with the FBI to gather evidence and build a case against the defendant.