LGT ON VECTOR FOR VISUAL APCH RECEIVED A TCASII RA ON AN LTT ON VISUAL APCH TO PARALLEL RWY. LTT WENT VISUALLY BELOW LGT IN CLOSE PROX.

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FLT WAS PROCEEDING UNDER STL RADAR APCH CTL. WX WAS VMC. WE WERE ESTABLISHED ON RWY 30R LOC AT 5000 FT. WE THEN RECEIVED A CLRNC TO DSND TO 3500 FT. AS THE ACFT DSNDED THROUGH APPROX 4000 FT; WE RECEIVED A TCASII RA TO 'CLB.' WHILE EXECUTING THE ESCAPE MANEUVER WE NOTICED A RED SYMBOL DIRECTLY UNDER THE ACFT SYMBOL ON THE IVSI WITH A SEPARATION NOTATION OF '00' FT. SHORTLY AFTER INITIATING OUR CLB; THE CAPT OBSERVED AN LTT Y SLOWLY DIVERGING FROM THE L SIDE OF OUR ACFT AND DIRECTLY ABEAM; BUT NOW BELOW US. AT ABOUT THE SAME TIME; WE RECEIVED A 'CLR OF CONFLICT' ADVISORY. WE IMMEDIATELY ADVISED THE CTLR WE HAD RECEIVED AN RA AND HAD COMPLIED WITH TCASII COMMANDS TO CLB. THE CTLR THEN ADVISED US WE HAD LTT Y TFC OFF OUR L ON VISUAL APCH TO RWY 30L AND THEN CLRED US FOR VISUAL APCH TO RWY 30R. THE REMAINDER OF THE APCH AND LNDG WAS UNEVENTFUL. AFTER LNDG THE CAPT CONTACTED A SUPVR AT STL TRACON AND WAS ADVISED THAT; AT THE TIME OF THE INCIDENT IN QUESTION; THE LTT Y HAD BEEN PROCEEDING ON A VISUAL APCH CLRNC TO RWY 30L. HE FURTHER STATED THAT THIS ACFT HAD RPTED US IN SIGHT AND HAD BEEN INSTRUCTED TO MAINTAIN VISUAL SEPARATION. PRIOR TO JOINING THE LOC; WE WERE ISSUED NO TA REGARDING THE LTT Y EVEN THOUGH WE WERE STILL UNDER RADAR CTL. ALSO; THE ATIS ADVERTISED VISUAL APCHS TO RWY 30L AND RWY 30R UTILIZING RWY 30R ILS AND RWY 30L LDA-DME. WHY DID THE LTT Y FLY TO THE R OF HIS VISUAL APCH COURSE? REALISTICALLY; WE CANNOT BE PRESUMED RESPONSIBLE FOR VISUAL SEPARATION FROM TFC WE WERE UNABLE TO SEE AND THAT WHICH WAS NOT RPTED TO US BY ATC.