FLC DEVIATES FROM TFC CTL ASSIGNED CLB TO DSND IN COMPLIANCE WITH TCASII RA REFING UNSEEN AND UNKNOWN TARGET.

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OUR ACFT WAS LEVEL AT 8000 FT; 250 KTS ON AN ASSIGNED HDG FROM DEP CTL WHEN 2 TARGETS APPEARED ON THE TCASII SCREEN APCHING FROM THE OPPOSITE DIRECTION. ATC ADVISED US OF TFC AT 11 O'CLOCK AND 500 FT BELOW US. TCASII SHOWED THIS ACFT AND AN ADDITIONAL ONE AT OUR 12:30 O'CLOCK AND 500 FT ABOVE US. ATC ISSUED US CLRNC TO CLB TO FL230. WE ACKNOWLEDGED THE CLRNC AND INITIATED THE CLB. AT INITIATION OF THIS CLB THE TCASII BEGAN A RAPID SERIES OF ALERTS WHICH CULMINATED WITH AURAL AND VISUAL INSTRUCTIONS TO 'DSND.' DSCNT WAS MADE TO 7500 FT FOR SEPARATION. AT 7500 FT THE CONFLICTING ACFT HAD EVIDENTLY MADE A RAPID DSCNT AS HE PASSED 1/2 MI TO OUR R AND NOW WAS DISPLAYED AS 500 FT BELOW US. ATC WAS ADVISED OF OUR DSCNT AS SOON AS THE RADIO WAS CLR. IT WAS ALMOST NONSTOP RADIO XMISSIONS DUE TO THE EXTREMELY HVY WORKLOAD FOR DEP CTL. AS THE CONFLICTING TFC PASSED TO THE R; THE OTHER TARGET ACFT WAS IDENTED VISUALLY BELOW AND TO OUR L. THE TFC TO THE R WAS NEVER IDENTED VISUALLY. AS THE TFC TO THE R DSNDED THROUGH OUR NEW FLT LEVEL; THE TCASII WARNING SWITCHED RAPIDLY TO 'CLB.' CLB TO FL230 WAS THEN REINITIATED. THIS ENTIRE HAPPENING TRANSPIRED IN A VERY SHORT TIME FRAME WHICH IS IMPOSSIBLE TO ESTIMATE AS ALL OF OUR ADRENALINE WAS 'QUITE ELEVATED.' IN ANALYZING THIS EVENT; WE SUSPECT THAT THERE WERE REALLY 2 ACFT APCHING US. THE ACFT TO OUR L WAS IN CONTACT WITH DEP CTL AND POSITIVELY IDENTED. HOWEVER; THE ACFT TO OUR R REMAINS UNKNOWN AND THIS WAS THE CAUSE OF OUR CONFLICT AND TCASII ALERT. FACED WITH A CERTAIN DEGREE OF CONFUSION BTWN ATC; WHO NEVER SAW THE ACFT TO OUR R AND THE FACT THAT WE ALSO COULD NOT VISUALLY IDENT THIS ACFT; WE FEEL THAT WE WERE LEFT WITH NO CHOICE OTHER THAN TO INITIATE THE TCASII COMMANDED INSTRUCTIONS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS FLYING AN L1011. THE EVENTS ALL HAPPENED VERY QUICKLY. THE RPTR IS NOT REALLY SURE WHICH OF THE 2 ACFT PROMPTED THE CLB RA. THE RPTR SAID HE HAD EXPERIENCED A FALSE TA BEFORE BUT NEVER A FALSE RA. THE FALSE TA MOVED ABOUT THE SCOPE SO RAPIDLY AS TO SUGGEST IT WAS FALSE. THE RA HE WAS SEEING BEHAVED VERY REALISTICALLY. THE BEHAVIOR OF THE AIRPLANE ON THE R DID NOT MIRROR THE BEHAVIOR OF THE AIRPLANE ON THE L. THE RPTR CONTACTED HIS CHIEF PLT WHO CONTACTED THE SALT LAKE TRACON FOLKS WHO WERE UPSET AT HIS DSNDING TOWARD THE ACFT BENEATH HIM AT 7500 FT AND HAD TO DEVIATE. SO FAR THE ISSUE IS APPARENTLY CLOSED.