SF34 RESPONDS TO TCAS RA. FLT CREW FEELS THE ADVISORY WAS ERRONEOUS.

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WE WERE LEVEL AT 9000 FT WHEN WE RECEIVED A TCAS 'TFC! TFC!' ALERT. I LOOKED AT THE TCAS DISPLAY; AND THE CONFLICT WAS INDICATED AT OUR 9-10 O'CLOCK POS; SAME ALT; 2-3 MI AWAY. IT WAS DISPLAYED IN YELLOW. AS WE DIRECTED OUR ATTN OUTSIDE TO LOCATE THE TFC; APPROX 3-5 SECONDS AFTER THE INITIAL TFC ALERT; WE RECEIVED A TCAS RA; 'DSND! DSND!' THE CAPT DISCONNECTED THE AUTOPLT AND BEGAN AN IMMEDIATE DSCNT PER THE TCAS INSTRUCTIONS. I NOTIFIED HOUSTON APCH THAT WE WERE DSNDING PER A TCAS RA. HOUSTON APCH INSTRUCTED US TO STOP OUR DSCNT BY 8000 FT; AS WE HAD TFC BELOW US. PER OUR MANUAL AND TRAINING; WE CONTINUED TO FOLLOW THE TCAS INSTRUCTIONS; AND I INFORMED HOUSTON APCH THAT WE WOULD CONTINUE OUR DSCNT PER THE TCAS RA. THE RA TOPPED APPROX 10 SECONDS AFTER THE 'DSND! DSND!' COMMAND; AND THE CAPT TOPPED THE DSCNT AND RETURNED TO 9000 FT. WE HAD DSNDED TO APPROX 8500 FT AT THE LOWEST POINT. I INFORMED HOUSTON APCH THAT THE RA WAS RESOLVED AND WE WERE RETURNING TO 9000 FT. I THEN LOOKED AGAIN AT THE TCAS DISPLAY; AND NOTICED 1 TARGET DIRECTLY BELOW US; INDICATING 2000 FT BELOW; TRACKING THE SAME DIRECTION AND IN THE EXPECTED LATERAL POS OF THE PREVIOUS CONFLICTING TARGET. NEITHER THE CAPT NOR I VISUALLY SAW THE OTHER ACFT. I DID NOT SEE ANY OTHER TARGETS ON OUR TCAS DISPLAY THAT COULD HAVE POTENTIALLY BEEN A CONFLICT. I DON'T HAVE ENOUGH KNOWLEDGE OF HOW THE TCAS SYS WORKS TO SAY EXACTLY WHY WE RECEIVED THE DSND COMMAND; BUT IT APPEARED TO ME THAT THE TFC WAS DISPLAYED TO US AT THE SAME ALT IN ERROR. THE TFC THAT CAUSED THE CONFLICT; ACCORDING TO THE SITUATION I GATHERED FROM HOUSTON APCH; WAS BELOW US BY AT LEAST 2000 FT WHEN THE RA OCCURRED. THIS COINCIDES WITH THE SITUATION WE SAW ON THE TCAS DISPLAY AFTER THE RA.