A B737-200 FLC RECEIVES A TCASII RA ON A XING HELI 200 FT BELOW THEIR ACFT AT 1500 FT; 3 MI OUT ON FINAL APCH TO RWY 13 AT LGA; NY.

Narrative

WE WERE APCHING LGA ARPT AT NIGHT AND RWY 22 ILS WAS THE PROC IN USE AS PER THE ATIS. DURING THE DSCNT; WE WERE OFFERED THE RIVER VISUAL APCH TO RWY 13. WE AGREED TO IT AND BRIEFED THE PROC. WE JOINED THE RIVER AND ONCE WE WERE ESTABLISHED IN THE LOC AND GS FOR RWY 13; AT APPROX 1500 FT MSL; THE CAPT (PNF) OBSERVED TFC ON TCASII AT 12 O'CLOCK POS; 200 FT BELOW US. I WAS LOOKING OUTSIDE AND RATHER FOCUSED IN THE MANEUVER SO I COULD NOT SEE THE TFC. THE CAPT NEVER SAW THE TFC VISUALLY; BUT RELYING ON TCASII HE TOLD ME TO STOP THE DSCNT. AT THAT POINT I STOPPED THE DSCNT AND SHORTLY AFTER THAT WE GOT AN RA FROM TCASII TO CLB. I APPLIED THRUST AND CLBED STRAIGHT OUT TO ABOUT 2200 FT MSL. WE RPTED TO THE TWR AND AT FIRST THEY SAID 'NOTHING OUT THERE' AND THEN THEY TOLD US A TARGET JUST POPPED OUT ON THEIR RADAR. AFTER THAT; WE WERE GIVEN VECTORS TO LAND ON RWY 22 UNEVENTFULLY. WE NEVER SAW THE TFC; BUT A PAX SITTING ON THE WINDOW CLAIMED SEEING TFC AT APPROX 200 FT PASS RIGHT UNDER US. I BELIEVE IT COULD HAVE BEEN A HELI CLBING OUT THE RIVER OR HOVERING ON TOP OF ONE OF THE BUILDINGS EQUIPPED WITH HELIPORT. BUT I'M JUST GUESSING. ATC WAS AS CONFUSED AS WE WERE THROUGHOUT THE EVENT IN REGARDS TO THE TFC'S ORIGIN AND PURPOSE.