ALTDEV ALT EXCURSION. THE FLC GOT A TCASII RA WHILE IN A DSCNT AND DURING THE MANEUVER THEY CLBED NEAR TO OTHER 'IN-SIGHT' TFC BEFORE RETURNING TO THEIR CLRNC ALT.

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CLRNC FROM ATL APCH WAS RECEIVED TO DSND FROM 6000 FT TO 4000 FT WHILE ON RADAR VECTOR TO RWY 27L AT ATL. AT APPROX 5500 FT TCASII ALERT 'TFC; 'TFC' FOLLOWED BY 'REDUCE DSCNT; REDUCE DSCNT;' FOLLOWED BY 'CLB; CLB.' WE FOLLOWED RA'S. WE INITIALLY DSNDED TO ABOUT 5300 FT THEN CLBED BACK TO APPROX 5500 FT AT WHICH TIME TFC WAS OBSERVED (A CIVIL JET) AT ABOUT 6000 FT MOVING R TO L. APCH QUESTIONED OUR CLB AND WE ADVISED WE WERE FOLLOWING OUR TCASII RA. WE HAD JUST PASSED OVER STONE MOUNTAIN ARPT WHEN THE ALERT OCCURRED. WE NEVER SAW ANY LOWER TFC. NO NEAR MISS OCCURRED; NO DRASTIC EVASIVE ACTION BECAME NECESSARY. APCH WAS VERY BUSY AND TO FOLLOW THE RA THERE WAS NO TIME TO ADVISE APCH. CTLR HAD NO FURTHER COMMENTS; NO OTHER ABNORMALITIES OCCURRED; UNEVENTFUL APCH WAS COMPLETED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING AN MD-11 FROM FRANKFURT TO ATL WHEN THE EVENT OCCURRED. NONE OF THE COCKPIT OCCUPANTS SAW THE INITIAL INTRUDER; BUT ALL OF THEM SAW THE ACFT AT 6000 FT. HE DESCRIBES THE SECOND ACFT AS A CPR JET WITH REAR MOUNTED ENGS. BASED ON RETURNS AND DATA FROM THE TCASII THE RPTR ESTIMATES THAT THE FIRST TARGET CAME PAST AT 800 FT VERT AND 1/2 MI HORIZ. THE SECOND TARGET CAME PAST AT 500 FT VERT AND 1 MI HORIZ. THE FIRST ACFT THAT STARTED THE WHOLE PROCESS IS SUSPECTED OF BEING A POP-UP FROM THE ARPT AT STONE MOUNTAIN; GA. THE RPTR HAS HAD ANOTHER RECENT TCASII EVENT IN THE SAME AREA (RPT ON THE WAY).