TFC ADVISORY GIVEN BUT WRONG TFC SIGHTED. TCASII RA CAUSED FLC TO CLB UNTIL TCASII ANNOUNCED CLR OF TFC.

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THE FOLLOWING TCAS INCIDENT OCCURRED ON A COMMERCIAL AIRLINE FLT INTO BDL ARPT IN AN MLG ACFT ON WHICH I WAS SERVING AS FO. WX CONDITIONS WERE VMC IN TYPICAL SUMMER HAZE RESULTING IN REDUCED VISIBILITY. WE WERE ON A VECTOR TO A L DOWNWIND LEG; ABOUT 20 MI FROM THE ARPT; AND HAD BEEN TOLD TO EXPECT A VISUAL APCH ONCE THE ARPT WAS IN SIGHT. DSNDING THROUGH ABOUT 7000 FT ON A CLRNC TO 6000 (CLEAN CONFIGN; 250 KTS IAS; AUTOPLT AND AUTOTHROTTLES ENGAGED; I WAS FLYING); WE RECEIVED A TFC ADVISORY FROM THE APCH CTLR CONCERNING AN ACFT AT OUR 11 O'CLOCK POS. THE TFC WAS DESCRIBED NOT BY ITS TYPE; BUT BY ITS OPERATOR'S NAME; A NAME NEITHER OF US RECOGNIZED. THE CAPT OF OUR ACFT SAW TFC 'A' THAT POS WHICH HE TOOK TO BE THE ACFT IN QUESTION AND HE ADVISED THE CTLR ACCORDINGLY. WHILE I DID NOT SEE THE TFC; THE CAPT ADVISED ME THAT AS LONG AS IT CONTINUED ON ITS PRESENT HDG; IT WOULD NOT BE A FACTOR FOR US. SHORTLY THEREAFTER; THE TCAS ADVISED 'TFC TFC' AND DISPLAYED A YELLOW CIRCLE AT OUR 11 O'CLOCK POS. THE TCAS WAS ON THE 40 MI RANGE AND DISPLAYED A LARGE NUMBER OF TARGETS (WELL IN EXCESS OF TEN). THE YELLOW TARGET; ON THIS RANGE; APPEARED VERY CLOSE TO OUR POS ON THE DISPLAY; SUFFICIENTLY CLOSE TO BE LOST IN CLUTTER OF OTHER ACFT AND ESSENTIALLY UNINTERPRETABLE; SO I SELECTED THE 5 MI RANGE FOR A MEANINGFUL LOOK. I ANTICIPATED THAT THE TARGET WOULD BE THE NON-CONFLICTING TFC WHICH HAD BEEN CALLED OUT AND SEEN BY THE CAPT. THE CAPT CONTINUED LOOKING OUT THE WINDSHIELD FOR THE TFC. BEFORE THE SHORTER RANGE DISPLAY COULD BE FULLY PAINTED AND INTERPRETED; THE TCAS GAVE US A 'CLB CLB CLB' ADVISORY AND THE PERIMETER LIGHTS ON THE VSI INDICATOR DISPLAYED RED FROM THE BOTTOM OF THE INST THROUGH ZERO AND UP TO SOME HIGH POSITIVE VERT CLB RATE; WHERE SEVERAL GREEN LIGHTS WERE DISPLAYED. MY EARLIER EXPECTATION THAT NO HAZARD EXISTED APPEARED NOW TO HAVE BEEN INCORRECT AND I RESPONDED BY ROLLING THE AUTOPLT'S VERT SPD THUMB WHEEL UP TO A HIGH RATE OF CLB. THE AUTOPLT'S SMOOTH TRANSITION FROM A 1000 FPM DSCNT TO A COMMANDED CLB IN EXCESS OF 2000 FPM WAS APPARENTLY NOT SUFFICIENT FOR THE SITUATION SINCE; IN THE MIDST OF THAT VERT SPD TRANSITION; THE TCAS ADVISED 'INCREASE CLB INCREASE CLB'. I DISENGAGED THE AUTOPLT AND HAND FLEW THE ACFT TO A HIGHER NOSE ATTITUDE. DURING THIS MANEUVER (WHICH CREATED A SLIGHT BUMP FOR THE PAXS SINCE THE PITCH TRIM HAD NOT QUITE KEPT UP WITH THE PITCH INCREASE); I SAW AN OPPOSITE DIRECTION MEDIUM SIZED TWIN PASS OFF TO OUR L SIDE AND SLIGHTLY BELOW US. I WOULD JUDGE THAT SEPARATION WAS LESS THAN HALF A MILE... PERHAPS A QUARTER OF A MI. AS IT DISAPPEARED FROM VIEW BEHIND US; THE TCAS ADVISED 'CLR OF CONFLICT'. I GENTLY TRANSITIONED FROM A HIGH CLB RATE TO A COMFORTABLE DSCNT AND RETURNED TO THE 6000 FT ALT TO WHICH WE HAD BEEN ORIGINALLY CLRED. THE CAPT ADVISED THE APCH CTLR OF THE INCIDENT AND THE CTLR REAFFIRMED THAT WE WERE CLRED TO RETURN TO 6000 FT. UPON SUBSEQUENT TELEPHONE CONVERSATIONS WITH THE CTLR; WE LEARNED THAT THE INTRUDING ACFT WAS A COMMUTER AND HAD BEEN THE TFC ORIGINALLY CALLED OUT TO US. THE TFC WE HAD SEEN HAD BEEN A SECOND TARGET (AN MLG) WHICH HAD NOT BEEN CALLED OUT. THE SUDDEN CLB HAD NOT; IN THIS INSTANCE; COMPROMISED SEPARATION WITH ANY OTHER IFR ACFT.