A B777 FLT CREW RECEIVE A TCASII RA AND HAVE AN NMAC WHILE DSNDING INTO LAX.

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ON THE EVENING OF JAN/SUN/04; I (FO) WAS ASSIGNED TO OPERATE A B777-200 WITH CAPT FROM DENVER TO LOS ANGELES INTL ARPT. UPON ARR IN THE LA BASIN WE WERE CLRED FOR THE CIVET FOUR ARR AND THE ILS TO RWY 25L. THE APCH WAS UNEVENTFUL TILL APPROX 5.5 MI FROM THE ARPT; JUST E OF THE LIMMA OM. AT THAT LOCATION WHILE DSNDING THROUGH 2000 FT MSL WE HAD 3 SIMULTANEOUS EVENTS OCCUR. 1) THE TCASII SYS SOUNDED AN RA ON A SMALL ACFT. 2) WE OBSERVED THE ACFT VISUALLY AT OUR 1 O'CLOCK POS AND LESS THAN 1 MI AT AN ALT OF 1500 FT SBOUND PARALLELING THE HARBOR FREEWAY. 3) THE LOS ANGELES TWR ADVISED US OF THE SAME TFC. I WAS FLYING THE ACFT AND REACTED QUICKLY AND SMOOTHLY TO THE RA AND STOPPED OUR DSCNT AT 1900 FT. THE UNIDENTED ACFT PASSED UNDER US WITH APPROX 400 FT VERT SEPARATION AND NO APPARENT LATERAL SEPARATION. ONCE THE RA WAS CONCLUDED; WE CONTINUED TO CONFIGURE THE ACFT; COMPLETE THE CHKLIST; STABILIZE THE APCH; AND COMPLETED AN UNEVENTFUL LNDG STRAIGHT-IN ON RWY 25L. ONCE CLR OF THE RWY; WE NOTIFIED GND CTL OF OUR INTENT TO FILE AN NMAC RPT. ONCE AT THE GATE; WE CONTACTED THE LOS ANGELES TWR AND COMPLETED THE PRELIMINARY NMAC RPT OVER THE PHONE AND NOTIFIED STATION DISPATCH OF THE EVENT. WE HAD A SHORT TELEPHONE DEBRIEFING WITH OUR DISPATCHER; THE DISPATCH MGR; AND THE COMPANY ATC COORDINATOR.