DEN83FA039

When a witness arrived at the arpt, the acft was at the fuel pumps & the plt had the preheater attached. The witness rprtd there was frost on the acft & the plt brushed the loose frost off with his hands. The plt asked for a window scraper, which was provided, but the witness did not know what he did with it. Aft refueling, the plt started the eng & remained on the ramp a short time bfr taxiing. A tkof was initiated from the 5300 ft rwy in calm wnd conds. The elev & temp were 9927 ft MSL & 2 deg f. reportedly, the acft rolled for more than 3/4 of the rwy length before bcmg airborne. The witness rprtd that after lift-off, the acft did not gain altitude, but remained low, bgn a lft turn, then went down. Another witness obsvd the acft fall from a low alt & crash in a lft wing low attitude. A pax rprtd the plane seemed to slow dwn & lose alt aft tkof, as if it was too hvy. However, disregarding any frost/ice, the acft was estimated 88 lbs blo the max wt lmt. No preimpact, mechanical failure were found with the airframe or eng.