LAX85MA012

The acft departed fullerton, CA (elev 96') at O958 PDT on a flt to big bear, CA (elev 6750'). There was no record of the plt obtaining a wx briefing. At aprx 1015, a witness heard the helicopter circling above the bottom of 'an early morning fog hanging over the ground at aprx 300 ft.' after circling about 3 to 5 min, the helicopter began to fly south when it suddenly was observed flying out of the fog at a steep angle toward the ground. It was angled nose down with a slight roll to the left. Subsequently, the helicopter crashed in a tail low, left bank attitude. The eng was reported to be operating at a hi power setting until the helicpoter crashed & burned. No preimpact part failure or malfunction was found. There was evidence of low rotor RPM, that the cyclic control was full aft & that the collective was full down at impact. Aprx 4 mi east, the 1019 wx was, in part: sky partially obscured, 1000' overcast, visibility 3/4 mi with fog.