OPERROR TCASII ALERT.

Narrative

CRUISING AT FL210; 98 NM NW OF STL ON THE UIN 143 DEG RADIAL; RIVERS 1 ARR TO STL. ZKC DIRECTED A DSCNT TO FL190. AS PNF ACKNOWLEDGED; PF INITIATED DSCNT. AS ACFT PASSED FL206; TCASII AUDIO OF 'TFC; TFC' OCCURRED. I CHKED THE TCASII DISPLAY AND NOTED THE TFC SYMBOL AT THE 0930 TO 10 O'CLOCK POS; APPROX 3 MI; 700 FT BELOW AND LEVEL. AS I LOOKED OUTSIDE IN AN ATTEMPT TO SIGHT THE TFC; THE TCASII SYS ISSUED A 'CLB; CLB' AUDIO RA AND THE DISPLAY INDICATED A MIN 2000 FPM CLB. THE ACFT WAS AT FL205 AT THIS TIME AND THE TFC SYMBOL ON TCASII WAS AT THE 9 TO 0930 O'CLOCK POS; 500 FT BELOW AND LEVEL; APPROX 2 1/2 MI. I TOLD THE PF THAT I DIDN'T HAVE SIGHT OF THE TFC AND HE IMMEDIATELY STARTED A CLB. I NOTIFIED CTR WE HAD AN RA ON THE TFC AT 9 O'CLOCK; WHICH I HAD SPOTTED AS I WAS TALKING TO THEM. THE ACFT WAS A TWIN ENG; PROP AT APPROX FL200. CTR CTLR DIRECTED US TO MAINTAIN FL210 AND A SHORT TIME LATER; ASKED IF WE HAD DSNDED BELOW FL210. I SAID THAT WE HAD REACHED FL205 BEFORE CLBING BACK TO FL210. THE TFC PASSED BEHIND US AT APPROX 2 NM AT THE CLOSEST POINT. SUPPLEMENTAL INFO FROM ACN 284951: WE PICKED UP THE ACFT VISUALLY AND DID NOT CONSIDER THE INCIDENT TO BE A NEAR MISS. LOWEST ALT WE WENT TO WAS 19600 FT. I THINK THE OTHER ACFT WAS AN IFR KING AIR AT FL200.