AN ACR DH8 FLC WAS INSTRUCTED BY THEIR OPS PERSONNEL TO USE A SECURED GATE TO DEPLANE THEIR PAX. UNFORTUNATELY; THE PAX BEING DEPLANED ORIGINATED FROM 2 DIFFERENT UNSECURED ARPTS. ALL PAX AND EMPLOYEES AFFECTED BY THIS ERROR HAD TO BE RESCREENED BY SECURITY.

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DEN OPS WAS ADVISED THAT OUR FLT WAS INBOUND FROM SHERIDAN AND GILLETTE; WY -- BOTH UNSECURED ARPTS. ON ARR AT THE RAMP; WE WERE ASSIGNED A GATE. THIS GATE WAS OCCUPIED SO OPS TOLD US TO HOLD OUR POS AND THEY WOULD SEND SOMEBODY OUT TO PARK US. AFTER APPROX 2 MINS; A RAMPER APPEARED AT AN ADJACENT GATE AND WAVED US IN. CAPT ASKED ME TO ADVISE PAX TO REMAIN SEATED UNTIL THE FASTEN SEATBELT SIGN WAS OFF. WHILE I WAS OFF THE FREQ; HE VERIFIED OUR GATE ASSIGNMENT TWICE VIA RADIO COM WITH RAMP OPS. ACFT WAS PARKED AND PAX RELEASED INTO CONCOURSE AT DEN. ONLY LATER DID I FIND OUT THAT THE GATE USED A 'SECURED' GATE. UNSCREENED PAX AND BAGGAGE HAD CONTAMINATED DEN. ALL OF THIS CONCOURSE'S FLTS AND ACR'S FLTS HAD TO RETURN; DEPLANE; AND ALL PAX AND EMPLOYEES HAD TO BE RESCREENED AT DEN. OUR CONCOURSE WAS LOCKED DOWN AND SECURED BY ARPT POLICE. THIS WAS MY FIRST DAY ON THE LINE AFTER COMPLETION OF IOE. I WAS TOTALLY UNFAMILIAR WITH DEN LAYOUT AND SECURITY PROCS AND WAS UNAWARE OF THE DIFFERENCE BTWN THE GATE ASSIGNED AND GATE USED. THE CAPT WAS FAMILIAR WITH THIS OP; BUT WAS SATISFIED AFTER CHKING TWICE WITH OPS THAT THEY HAD A PLAN IN PLACE FOR THESE PAX AND THEIR BAGGAGE. THIS WAS THE FIRST DAY OF THE TRANSITION BTWN MY COMPANY AND ANOTHER'S OP OF THE DEN SYS. OUR AIRPLANES AND CREW ARE WET-LEASED TO THE OTHER OP WHICH PROVIDES ALL GND SUPPORT. FOR MANY OF THE GND PEOPLE IT WAS THEIR FIRST DAY AND THEY TOO HAD NOT RECEIVED THOROUGH TRAINING ON THE DIFFERENCES BTWN SECURED AND UNSECURED FLTS. THE RESULT WAS AN ALMOST COMPLETE SHUTDOWN OF DEN FOR 4 HRS. ALL OF THIS WAS; IN MY OPINION; A RESULT IN THE BREAKDOWN OF COMS AND A LACK OF SUPERVISION OF GND OPS. THE REGIONAL AIRLINE OP IS VERY FAST PACED; ESPECIALLY WITH 10 AND 15 MIN TURN TIMES OF ACFT. ALL EMPLOYEES ARE HURRIED TO PARK; DEPLANE; ENPLANE; AND DEPART ASAP. SITS LIKE THIS ONE WILL CONTINUE TO OCCUR AS LONG AS SOME ARPTS ARE UNSECURED. OUR COMPANY IS ADDING A LINE TO OUR DISPATCH/RELEASE THAT MUST BE SIGNED BY CAPT AND GND PERSONNEL STATING ACFT IS SECURE/UNSECURE. GND OPS WILL RETRAIN AND RERAMP INBOUND/OUTBOUND ACFT DESIGNATION AND COM SYS.