ACFT 1 INBOUND TO THE ARPT RECEIVED A TCASII ALERT REQUIRING EVASIVE ACTION TO BE TAKEN FROM ACFT 2.

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INBOUND TO BWI ARPT; APPROX 20 MI SE OF THE ARPT. INSTRUCTED TO PROCEED DIRECT TO THE ARPT; MAINTAIN 4000 FT; RPT THE FIELD IN SIGHT FOR VISUAL APCH. NUMEROUS ACFT IN AREA (VFR AND IFR). AT THIS POINT; TCASII ISSUES AN RA WITH INSTRUCTIONS TO 'CLB.' THE CAPT PICKED UP THE CONFLICTING ACFT VISUALLY AND EVASIVE ACTION WAS TAKEN. I (FO) WAS FLYING AND WAS VISUALLY ABLE TO AVOID THE CONFLICTING ACFT. THE CONFLICTING ACFT WAS AN ACR B-727. THE B-727 WAS AT THE SAME ALT (4000 FT MSL) AND WAS TURNING TOWARD THE ARPT FROM OUR 10-11 O'CLOCK POS; SUBSTANTIALLY FASTER THAN OUR ACFT. WE WERE APPROX AT 200 KTS AT THE TIME. NO TFC SEPARATION OR WARNING OF A CONFLICT WAS GIVEN BY APCH CTL FROM THIS ACFT. WE WERE; AT BEST ESTIMATE; BTWN 500-1000 FT FROM THE B-727. WE VISUALLY IDENTED THE ARPT AND WERE CLRED FOR THE VISUAL APCH TO RWY 33R. THE B-727 PROCEEDED TO RWY 33L WITHOUT ANY MORE CONFLICTS. AN UNEVENTFUL LNDG WAS MADE AT BWI. UPON ARR AT THE TERMINAL; THE CAPT CALLED THE APCH CTL AND INFORMED THEM OF THE OCCURRENCE. HE WAS TOLD THAT IT WOULD BE LOOKED INTO.