A DC-9 FLC RECEIVED A TCASII RA ALERT FOR 2 ACFT AND DSNDED 400 FT IN RESPONSE. ATC STATED SEPARATION LOST WITH CONFLICTING ACFT.

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AFTER FLYING THE ALAMO 2 SID FROM SAT (LUFKIN TRANSITION); WE WERE CLRED DIRECT TO LUF WHILE STILL SHORT OF ELP. WE APPROX PARALLELED THE LUF TRANSITION. IN THE VICINITY OF LUF; WE WERE CLRED TO FL330 WHILE CLBING THROUGH THE MID-TWENTIES. AT APPROX FL275; WE WERE ADVISED TO MODIFY OUR LEVEL OFF AND STOP THE CLB AT FL290 DUE TO TFC ABOVE. AS WE CRUISED N OF LUF AT FL290; WE WERE ADVISED OF WBOUND TFC; A CITATION; AT FL280. SINCE THE CITATION ADVISORY WAS TO OUR R; I BEGAN TO SCAN THE SKY FOR IT. WE ABRUPTLY HAD A TCASII DSCNT ADVISORY REQUIRING A 1500-2000 FPM (GREEN ZONE) ADJUSTMENT. A QUICK GLANCE AT THE VSI SHOWED 2 BLIPS. THE CAPT BEGAN HIS DSCNT AS I SAW THE CITATION AT OUR 3 O'CLOCK POS. AS I WATCHED THE CITATION; WHO NEVER TURNED; THE TCASII ADVISED 'MONITOR VERT SPD.' I NEVER SAW THE ALTIMETER DECREASE BELOW 28600 FT MSL. AS THE CAPT BEGAN HIS CLB BACK TO FL290; THE CTR ADVISED US TO 'MAINTAIN FL290.' I THEN INFORMED THEM OF THE TCASII ADVISORY TO DSND. THEY SAID SEPARATION HAD BEEN LOST AND GAVE A PHONE NUMBER. CTR ALSO STATED NO OTHER TFC WAS IN THE AREA; BUT I STILL BELIEVE I SAW 2 BLIPS ON THE TCASII. THIS ALL HAPPENED IN A 10-15 SECOND TIME PERIOD. THE CAPT'S ACTIONS IN RESPONSE TO THE TCASII WERE CORRECT. IF WE LOST SEPARATION; ATC MUST HAVE ALLOWED THE CITATION TOO CLOSE LATERALLY. I CANNOT DETERMINE WHY WE HAD A DSCNT ADVISORY IF THERE WAS NOT AN ACFT ABOVE US.