THE ACFT WAS CHARTERED TO TAKE UNIVERSITY (WWU) PSNL TO AN ALUMNI DINNER. WHILE RTRNG AT NGT, THE PLT MADE A LOCALIZER ONLY, ILS RWY 16 APCH IN MARGINAL VFR CONDS. DRG THE APCH, RADAR SVC WAS TERMINATED & THE PLT WAS TOLD TO CTC BELLINGHAMRADIO, BUT HE DID NOT LND OR CTC THE FACILITY. RADAR DATA SHOWED HE CONTD SOUTHBOUND TWD WWU & CHGD THE TRANSPONDER CODETO 1200 (VFR). RADAR CTC WAS LOST FOR APRX 1-1/2 MIN AS THE ACFT PASSED OVR THE VCNTY OF WWU & 1 OF THE PAX'S HOME, THENRADAR CTC WAS REESTABLISHED. WITNESSES IN THE AREA RPRTD SEEING A LOW FLYING ACFT AT APRX THAT TIME. THE ACFT THEN FLEW TO THE VCNTY OF SVRL LARGE REFINERIES APRX 9 MI NNW OF THE ARPT & RADAR CTC WAS LOST AGAIN. WHEN THE ACFT DID NOT LAND AT THE ARPT, A SEARCH WAS STARTED. IT WAS FND WHERE IT HAD HIT 200' TREES & CRASHED. NO ONE SAW THE ACDNT, BUT 2 PERSONSSAW AN ACFT FLYING AT LOW ALT IN THAT VCNTY WITH THE LNDG LGT ON. THEY ALSO RPRTD PTCHY FOG IN THE AREA. SUBSEQUENTLY, THE ACFT TURNED TO AN EASTERLY HDG BFR IT CRASHED. NO PREIMPACT MECH PRBLM WAS FND THAT WOULD HAVE CAUSE THE ACDNT.