NMAC BTWN AN A320 IN CRUISE AND A TAIL MOUNTED 3 ENG EXECUTIVE ACFT JUST STARTING TO DSND AS PREVIOUSLY INSTRUCTED BY ATC. ATC ADVISED RPTR THAT THE OTHER ACFT WAS SLOW IN STARTING DSCNT AND SHOULD HAVE BEEN WELL BELOW RPTR'S ACFT.

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OUR FLT WAS FROM IND TO DTW AT FL210; ASSIGNED 290 KIAS; APCHING FWA VOR. OUR TCASII POPPED UP AN ADVISORY 8 MI AHEAD; +2 (200 FT ABOVE US); 12:30 O'CLOCK POS. WITHIN A FEW SECONDS; THE TCASII WENT FROM ADVISORY TO WARNING; DIRECTING A DSCNT. IMMEDIATELY; I CLICKED THE AUTOPLT OFF AND BEGAN A DSCNT. THE INTRUDER STILL SHOWED +2 ABOVE US BUT CLOSER. THE TCASII; WHICH HAD BEEN 1000 FT DSCNT; WAS NOW SHOWING ALL RED ON OUR VVI SO I KEPT PUSHING THE NOSE DOWN. VISUAL SCANS OUTSIDE WERE NOT SHOWING AN ACFT. THE CAPT HAD TOLD ATC WE WERE RESPONDING TO A TCASII. I RECALL THAT THEY WERE TELLING ANOTHER ACFT TO DSND. BY THE TIME WE WERE AT 2500 FT VVI DSNDING; THEY TOLD US TO CLB ABOVE FL210. I VISUALLY ACQUIRED THE OTHER ACFT AT OUR 12:30 O'CLOCK POS. IT LOOKED LIKE HE WOULD PASS ABOVE US. THIS WAS CONFIRMED ON THE TCASII. THE ACFT; WHICH HAD APPEARED TO BE A 3 ENG TAIL MOUNTED CORPORATE ACFT; PASSED TO THE L AND ABOVE. OUR ALT WAS FL193 WHEN WE BEGAN LEVELING OFF. I WISH I KNEW THE ELAPSED TIME -- SOMEWHERE BTWN 12-18 SECONDS. ATC LATER INFORMED US THAT THE OTHER ACFT HAD BEEN GIVEN DIRECTIONS TO DSND BUT WAS SLOW IN DSNDING. I SUSPECT THAT IT WAS NOT TCASII EQUIPPED BECAUSE IT CONTINUED TO DSND WITH AND INTO US.