United States Marshals Service
federal law enforcement agency of the United States
The U.S. Marshals Service does law enforcement for federal courts. It is mainly responsible for protecting court officers like judges in court buildings.
It is also responsible for finding fugitives, serving arrest warrants, and prisoner transport.
It is different from the FBI, which is responsible for things like counter-terrorism and going after organized crime, and which operates in multiple countries, not just the USA.