FLC OF AN MLG STOPPED THEIR CLB AND DSNDED IN RESPONSE TO A TCASII RA CAUSED BY A CONVERGING TARGET COMING AT THEM AS THEY CLBED. ATC ANSWERED AFFIRMATIVELY WHEN QUESTIONED BY THE CAPT REGARDING IF ATC KNEW OF THE INTRUDER ACFT.

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DURING CLB TO 23000 FT NEAR CENTRALIA VOR; WHILE PASSING THROUGH APPROX 14500 FT; THE TCASII ALERTED US TO TFC CONVERGING RAPIDLY FROM ABOUT 2 O'CLOCK POS. IT INDICATED THE ACFT WAS 900 FT ABOVE US. I IMMEDIATELY INSTRUCTED THE FO (WHO WAS FLYING) TO 'STOP THE CLB.' THE TCASII WENT FROM 'TFC' TO 'MONITOR VERT SPD' TO 'DSND; DSND' IN RAPID PROGRESSION. THIS TFC WAS NOT RPTED BY ATC. IN RESPONSE TO THE TCASII RA; THE CONVERGENCE RATE AND ALT CLOSURE; THE FO APPROPRIATELY CLOSED THE THROTTLES AND PUSHED OVER. I ASKED ATC IF THEY SHOWED TFC TO OUR R. ATC RESPONDED WITH WHAT SOUNDED LIKE 'AFFIRMATIVE; VFR FIFTEEN FIVE.' I DID NOT OBSERVE THE FINAL XING ALT. WE DID NOT VISUALLY SIGHT THE ACFT. DURING THE PUSH OVER IN RESPONSE TO THE RA; 2 PAX WERE LIFTED FROM THE FLOOR WHILE RETURNING FROM A LAVATORY. NO INJURIES OCCURRED. I INFORMED ATC 'WE HAD A TCASII ALERT ON THAT ACFT.' THEY RESPONDED 'ROGER.' I INFORMED HIM 'WE HAD TO STOP THE CLB AND DSND TO AVOID THAT TFC.' HE RESPONDED 'ROGER' AND GAVE US A FREQ CHANGE. MY IMPRESSION WAS THAT THE CTLR WAS AS STARTLED AS WE WERE. HIS RESPONSES WERE SHORT AND TIGHT. THOUGH I NEVER SAW THE TFC; DUE TO THE CLOSURE RATE AND RELATIVELY HIGH ALT I SUSPECTED IT WAS POSSIBLY A MIL ACFT. SOLUTION? POSSIBLY BETTER ATC VIGILANCE. IF IT WAS A MIL ACFT BETTER COMS WITH ATC ABOUT ALT AND INTENTIONS.