DESPITE ADVISORIES FROM THE TOWER; C172 ENTERS FLIGHT PATH OF CRJ ON VISUAL APCH. NMAC RESULTS.

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THE LOCAL TWR ADVISED US THAT THERE WAS A C172 OPERATING IN THE VICINITY OF THE ARPT AT 1000 FT MSL. THE TWR ASKED IF WE HAD VISUAL CONTACT OF THE ACFT. I RESPONDED THAT WE DID NOT HAVE VISUAL CONTACT. TWR INFORMED THE C172 THAT AN RJ WOULD ENTER A R DOWNWIND FOR RWY 32 AND TO STAY CLEAR OF THE AIRSPACE SURROUNDING THE RWY. THE PLT OF THE C172 ACKNOWLEDGED THE REQUEST. AS THE CAPT MADE HIS R TURN TO BASE; I CONTINUED TO SCAN FOR THE ACFT WITHOUT SUCCESS. I WAS ABLE TO MONITOR HIS POS ON THE TCAS BUT FAILED TO MAKE VISUAL CONTACT. AS THE CAPT TURNED FINAL; THE TWR REMINDED THE C172 TO STAY CLEAR OF THE APCH END TO RWY 32. THE ACFT NO LONGER APPEARED ON THE TCAS AND WE WERE FULLY CONFIGURED AND CLRED TO LAND. AT APPROX 1400 FT MSL I SAW THE C172 (WHITE WITH RED TRIM) MAKE A L TURN AND CREATED A HEAD-ON EVENT. THE TCAS INDICATED HIS POS WAS 1200 FT MSL AND GAVE AN IMMEDIATE CLB RA. THE CAPT AND I SIMULTANEOUSLY SAID 'GO AROUND' AND EXECUTED THE GAR PROFILE. IN ADDITION TO THE PROFILE; THE CAPT TURNED SEVERAL DEGS TO THE L TO CREATE MORE SPACE BTWN THE ACFT. I NOTIFIED THE TWR THAT WE EXECUTED A GAR AND ASKED WHAT ALT HE WOULD LIKE US TO RE-ENTER THE TFC PATTERN. WE WERE ASKED TO MAKE R TFC AT 2000 FT MSL BACK TO RWY 32. AFTER COMPLETING THE WALKAROUND; THE CAPT INFORMED ME THAT HE SPOKE WITH ONE OF THE CTLRS AND THEY HAD ALREADY CONTACTED THE LOCAL FSDO ABOUT THE EVENT. IT APPEARED TO ME THAT THE C172 WAS APPROX 250 FT FROM OUR ACFT. THE CAPT BELIEVED THE DISTANCE TO BE CLOSER TO 150 FT.