Simplify the following text for a Grade 12 native English-speaking person.
Original: The Chicago Police Department , also known as the CPD , is the principal law enforcement agency of the City of Chicago , Illinois , in the United States , under the jurisdiction of the city mayor .
Simplified: The Chicago Police Department , also known as the CPD , is the main law enforcement agency of the City of Chicago , Illinois , in the United States , under the jurisdiction of the city mayor .