WHILE ON APCH AND CLOSE TO THE OM; RPTR ACFT HAD TCASII RA; CONFIRMED INTRUDER DSNDING ON TOP OF THEM; RPTR TOOK EVASIVE ACTION AND DSNDED. INTRUDER APPARENTLY ALSO RECEIVED RA AND CORRECTED BY CLBING. NEITHER TWR NOR APCH CTL ADVISED OF CONFLICTING TFC. THE INTRUDING ACFT WAS POSSIBLY A RWY 24L-R VISUAL APCH THAT OVERSHOT THE FINAL APCH.

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WE WERE CLRED FOR A CIVET 1 ARR; ASSIGNED SPD 320 KIAS. AFTER PASSING CIVET AT APPROX XA40 PDT; WE WERE CLRED FOR AN ILS RWY 25L APCH; ASSIGNED SPD 250 KIAS. IN THE VICINITY OF THE FUELR INTXN WE WERE INSTRUCTED TO SLOW TO 220 KIAS; BECAUSE APCH CTL WAS TURNING AN L1011 AHEAD OF US. WHEN WE HAD A VISUAL ON THE L1011 WE WERE INSTRUCTED TO SLOW TO 190 KIAS AND FOLLOW THE L1011 TO RWY 25L. AT APPROX 10 MI (3000 FT MSL) WE GOT A TA WITH A TARGET AT 2-3 O'CLOCK. WITHIN 10 SECONDS WE GOT AN RA WITH A 'DSND COMMAND;' QUICKLY FOLLOWED BY 'INCREASE DSCNT RATE' COMMAND. ALMOST SIMULTANEOUSLY; WE SAW AN ACFT IN A STEEP DSNDING R-HAND TURN BELLY UP TO US AT APPROX 1500-3000 FT LATERALLY (APPROX 500 FT ABOVE OUR POS). I TOOK IMMEDIATE VISUAL EVASIVE ACTION AND INCREASED MY RATE OF DSCNT BELOW THE GS TO AVOID A COLLISION. THE THREAT ACFT APPEARED TO BE CORRECTING HIS DIVE INTO A CLB ( I ASSUME HE WAS REACTING TO HIS CORRESPONDING RA CLB COMMAND). AFTER RECEIVING A 'CLR OF CONFLICT' WE CONTINUED OUR APCH TO AN UNEVENTFUL LNDG. AT NO TIME DID WE RECEIVE ANY VHF RADIO WARNING OF THE THREAT ACFT (THE IMPENDING MIDAIR COLLISION).