CAPT OF A B757-200 DSNDED IN RESPONSE TO A TCASII RA CAUSED BY A LEARJET LEVEL AT 14000 FT.

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WE ENCOUNTERED A NEAR MISS WITH A LEARJET; ON DSCNT INTO SANTA ANA; CA. FLT WAS ON THE EASTSIDE THREE ARR AND HAD JUST CROSSED BANDS INTXN LEVEL AT 15000 FT. WE WERE TALKING TO SOCAL APCH AND HAD BEEN ISSUED CLRNC TO DSND AND MAINTAIN 10000 FT. THE FREQ WAS CONGESTED WITH THE REROUTING OF A GA ACFT BY APCH CTL; AND THE CTLR'S ATTN WAS DIVERTED TO THE ISSUING AND READBACK OF THE CLRNC. THE READBACK WAS IN PROGRESS DURING THE ENTIRE INCIDENT. AT APPROX 14500 FT WE RECEIVED A 'TFC; TFC' TCASII ADVISORY. A YELLOW CIRCLE WITH NO ALT DATA POPPED UP ON THE MAP DISPLAY INDICATING A TARGET APPROX 45 SECONDS AWAY FROM OUR ACFT. PASSING THROUGH 14200 FT THE TARGET XMITTED ALT DATA FOR THE FIRST TIME AND SHOWED -200 FT. ALMOST IMMEDIATELY AND APPROX 35 SECONDS FROM OUR ACFT THE TARGET TURNED TO A RED SQUARE AND A 'DSND; DSND' RA WAS ISSUED BY THE TCASII. I IMMEDIATELY EXECUTED AN AGGRESSIVE TCASII ESCAPE MANEUVER. APPROX 25 SECONDS FROM THE FLT PATH XING POINT WE HAD 500 FT VERT SEPARATION. 15 SECONDS FROM THE FLT PATH XING POINT I VISUALLY ACQUIRED THE ACFT AND COULD CONFIRM IT WAS LEVEL AT 14000 FT. WE PASSED UNDERNEATH THE ACFT WITH APPROX 1000 FT OF VERT SEPARATION. THE OTHER ACFT WAS A LEAR MODEL 31 OR 61 WITH WINGLETS. I DO NOT RECALL NOTICING THE COLOR. 15 SECONDS AFTER THE INCIDENT THE FREQ WAS CLR FROM THE GA REROUTE; AND WE NOTIFIED SOCAL APCH OF THE NEAR MISS. THE CTLR REPLIED 'OH THAT'S A LEARJET; HE IS VFR AT 14000 FT.' HIS TONE INDICATED THAT LEARJET WAS NOT HIS RESPONSIBILITY; SINCE HE WAS VFR. WITHOUT THE TCASII ESCAPE MANEUVER THERE WAS A HIGH PROBABILITY OF A MIDAIR COLLISION. SEVERAL FLT ATTENDANTS RPTED FLOATING SEVERAL INCHES ABOVE THE FLOOR DURING THE INCIDENT. ALL PAX WERE SEATED AND NO INJURIES WERE RPTED.