Narrative:

On 45 mi final to runway 25L at lax, we were established on localizer and GS, on speed and cleared 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 aircraft. As I looked outside for the target, we got an RA commanding a 2000-2500 FPM climb. The first officer (PF) promptly complied, we informed tower and at this point were too close and high to continue the approach, and executed a missed approach, leveling at 2000 ft. We were vectored back around for a second uneventful approach and landing. We never saw the other aircraft, if there was one. In subsequent conversation with tower supervisor at lax, he informed me that 3-4 people in the tower had neither a visual nor electronic identify of any intruder aircraft. He did say that there was an aircraft in the hawthorne airport pattern at 1200 ft, which would fit. The question that comes up is that he said that aircraft was 1 - 1 1/2 mi south of us. Our TCASII screen showed our target superimposed on us, with the 5 mi scale selected. It seems unlikely that all the tower personnel could miss an intruder aircraft. At this point we're not sure what, if anything, was below us. What are the possibilities, if any, for a false target/RA?

Related reports:

Google
 

Original NASA ASRS Text

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

Narrative: 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?

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.