A DO328 FLT CREW IS CLRED FOR THE VISUAL APCH AND ENCOUNTERS A CONFLICT WITH A B737.

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WE WERE BEING VECTORED FOR A VISUAL APCH TO RWY 32 IN PITTSBURGH. WHILE ON A 090 DEG HDG; BEING VECTORED FOR THE APCH; WE CALLED THE ARPT IN SIGHT. APCH CLRED US TO 2500 FT AND CLRED US FOR THE VISUAL APCH TO RWY 32. A B737 APPEARED AT OUR ALT AND LESS THAN 1 MI HORIZ DISTANCE. WE RECEIVED AN RA; STATING 'STOP DSCNT.' OUR CAPT (THE PF FOR THE LEG) BANKED R TO AVOID THE B737. IT BANKED R AT THE SAME TIME. ATC THEN ASKED IF WE HAD THE ACFT IN SIGHT; GAVE US A 050 DEG HDG AND TOLD US TO CONTACT THE TWR; CLRED FOR THE VISUAL RWY 32. CONTRIBUTING FACTORS: WE WERE NEVER ADVISED OF THE POS OF THE ACFT WE WERE FOLLOWING UNTIL IT WAS TOO LATE. THE TCASII DID NOT SHOW US THE B737 UNTIL IT SUDDENLY GAVE US AN RA; IT WAS IN BELOW MODE. THE B737 WAS FINALLY SHOWN AT OUR ALT AND LESS THAN 1 MI AWAY. WHILE WE HAD VISUAL CONTACT WITH THE ARPT; WE WERE IN HAZE AND COULD NOT SEE THE B737. CORRECTIVE ACTIONS: ATC SHOULD NOTIFY US OF THE POS OF THE ACFT WE ARE FOLLOWING ON A VISUAL APCH. TCASII SHOULD HAVE GIVEN US A TA BEFORE THE RA.