As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PRELIMINARY - AIRCRAFT DEPARTED ON MEDICAL EVACUATION TRIP. AIRCRAFT DID NOT PRESSURIZE. PIC NOTED LACK OF PRESSURIZATION BY THE EMERGENCY OXYGEN MASK AUTOMATIC DEPLOYMENT. AIRCRAFT AIR TURNED BACK. AIRCRAFT REMOVED FROM SERVICE. PERFORMED MM 21-30-00, PREFLIGHT TEST SATISFACTORY. AIRCRAFT REMAINS OUT OF SERVICE PENDING FURTHER TROUBLESHOOTING.