Check the 3rd item in the posting at discussions.flightaware.com/view … icao+codes
This shows all of the ICAO codes. These form the 1st 2 letters of the airport code. If you notice, the USA actually has 7 different codes:
K: 48 contiguous states
PB: Baker Island
PJ: Johnston Island
PW: Wake Island
Baker Island’s airport is abandoned, Johnston Island’s airport is closed and not maintained, Midway has 3 airport but only one is operational, and Wake Island has 1 airport.
Wake Island and United States Pacific Island Wildlife Refuges entries in the The World Factbook, published by the CIA (yes, that one, not the other one.