TCASII RA FLOWN ON FINAL APCH TO LAX WITH NO TFC IN THE VICINITY ACCORDING TO TWR OR SIGHTED BY THE CREW.

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ON 45 MI FINAL TO RWY 25L AT LAX; WE WERE ESTABLISHED ON LOC AND GS; ON SPD AND CLRED TO LAND. WE RECEIVED A TCASII TA; AND WHEN I LOOKED AT THE SCOPE (5 MI RANGE SELECTED) WE HAD A RED TARGET; 100 FT LOW; DIRECTLY UNDER OUR ACFT. AS I LOOKED OUTSIDE FOR THE TARGET; WE GOT AN RA COMMANDING A 2000-2500 FPM CLB. THE FO (PF) PROMPTLY COMPLIED; WE INFORMED TWR AND AT THIS POINT WERE TOO CLOSE AND HIGH TO CONTINUE THE APCH; AND EXECUTED A MISSED APCH; LEVELING AT 2000 FT. WE WERE VECTORED BACK AROUND FOR A SECOND UNEVENTFUL APCH AND LNDG. WE NEVER SAW THE OTHER ACFT; IF THERE WAS ONE. IN SUBSEQUENT CONVERSATION WITH TWR SUPVR AT LAX; HE INFORMED ME THAT 3-4 PEOPLE IN THE TWR HAD NEITHER A VISUAL NOR ELECTRONIC IDENT OF ANY INTRUDER ACFT. HE DID SAY THAT THERE WAS AN ACFT IN THE HAWTHORNE ARPT PATTERN AT 1200 FT; WHICH WOULD FIT. THE QUESTION THAT COMES UP IS THAT HE SAID THAT ACFT WAS 1 - 1 1/2 MI S OF US. OUR TCASII SCREEN SHOWED OUR TARGET SUPERIMPOSED ON US; WITH THE 5 MI SCALE SELECTED. IT SEEMS UNLIKELY THAT ALL THE TWR PERSONNEL COULD MISS AN INTRUDER ACFT. AT THIS POINT WE'RE NOT SURE WHAT; IF ANYTHING; WAS BELOW US. WHAT ARE THE POSSIBILITIES; IF ANY; FOR A FALSE TARGET/RA?