ALTDEV ATX X UNAUTH DSCNT BELOW ASSIGNED ALT DURING DSCNT.

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ATX X DSCNT FROM 8000 FT TO 6000 FT INTO KILLEEN; TX; GRAY APCH CALLED CONVERGING TFC CLBING THROUGH 6000 FT. NEITHER THE CAPT NOR MYSELF HAD A VISUAL ON THE TFC AND I WAS WORRIED ABOUT THE TFC ENDING UP AT THE SAME ALT AS US. I CONTINUED THE DSCNT TO 5500 FT WHEN THE CAPT REMINDED ME OF THE 6000 FT CLRNC. I CLBED BACK TO 6000 FT WITH NO FURTHER INCIDENT. I BELIEVE THE ALTDEV WAS CAUSED BY MY DISTR IN LOOKING FOR THE TFC. THE ONLY REMEDY I CAN THINK OF WOULD BE AN ALT ALERTER AS WELL AS BETTER ATTN ON MY PART. SUPPLEMENTAL INFO FROM ACN 294063: ON OUR DSCNT INTO ILE; AND APPROX 400 FT BEFORE OUR LEVELOFF ALT; GRAY APCH CALLED CONFLICTING TFC VERY CLOSE TO US AND CONVERGING ON OUR FLT PATH. BOTH MY ATTN AND THE PF'S ATTN WERE DIVERTED TO FIND THE TFC. WHILE WE WERE LOOKING FOR TFC WE DSNDED APPROX 500 FT BELOW OUR ASSIGNED ALT AND THEN IMMEDIATELY RECOVERED BACK TO ASSIGNED ALT. IF WE HAD AN ALT ALERTER AND THE PF HAD REMAINED ALERT TO FLYING THE PLANE; THIS PROBABLY WOULD NOT HAVE HAPPENED.