COMMUTER LTT HAS TCASII TARGET ON COLLISION COURSE. TAKES EVASIVE ACTION.

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WHILE HOLDING AT THE DAY VOR ON THE 270 DEG RADIAL INBOUND; A TARGET WAS OBSERVED ON OUR TCASII AT 10 O'CLOCK AND 5 MI (TCASII WAS IN THE 5 MI MODE). TARGET WAS NOT CALLED TO OUR ATTN BY ATC; TARGET WAS SHOWING PLUS 300 FT ABOVE OUR CURRENT HOLDING ALT OF 10000 FT (ALTIMETER SET TO 29.67). TFC CONTINUED TO APCH OUR POS. WHEN AT OUR 2 MI RANGE; MY REQUEST TO MY COPLT WAS TO VERIFY WITH DAYTON APCH THAT TFC WAS APCHING US AT 10300 FT. THE FREQ AT THAT MOMENT WAS CONGESTED. TFC NOW INDICATED PLUS 200 FT ABOVE OUR ALT. TCASII ALERTED 'TFC.' IN NOT BEING ABLE TO VERIFY OUR TARGET I DECIDED TO TAKE EVASIVE ACTION; A 60 DEG R DSNDING TURN TO 9500 FT. WE TOLD ATC OF OUR DEV AFTERWARD AND WE WERE TOLD TFC DID EXIST AT OUR POS AT 10300 FT. LATER HE HAD CLBED TO 10500 FT WE WERE ALSO TOLD HE WAS VFR AND DAYTON APCH WAS NOT TALKING TO HIM AT THE TIME (THE CLOUD TOPS WERE AT 15000 FT AND WE WERE SOLID IMC). LATER WE WERE INFORMED THAT OUR CONFLICTING ACFT HAD PICKED UP AN IFR CLRNC AND THEY WERE GOING TO FOLLOW UP WITH TRACKING THIS ACFT AS OF NOW. I DON'T KNOW WHEN I WILL HEAR FROM THEM. AS OF YET NO ACTIONS HAVE BEEN TAKEN; TO MY KNOWLEDGE.