As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ACFT CLIMBED TO FL200 AND HEARD A LOUD POP AND A LOUD RUSH OF AIR COMING FROM MAIN CABIN DOOR. STARTED TO SEE A DROP IN CABIN PRESSURE, PILOTS THEN DESCENDED AND DIVERTED BACK TO ORIGINATING AIRPORT. PILOTS DID DECLARE AN EMERGENCY, ACFT LANDED WITH NO PROBLEMS. WE ISSUED A SPECIAL FLIGHT PERMIT AND FLEW TO SEVICE CENTER, REPAIRS MADE AT SERVICE CENTER. APPEARED THAT THE SEAL PULLED OUT OF CHANNEL.