**This report was modified on 4/21/2016. Please see the public docket for this accident for additional information.** The instrument-rated commercial pilot was executing a circling VOR/DME instrument approach in IFR conditions. The pilot reported the runway in sight and was circling to land west of the airport just before the accident. Witnesses reported that the airplane descended in a near vertical attitude from the overcast, striking a highway curb lane about .75 miles from the airport. No evidence of preimpact failures or malfunctions with the primary flight controls or engines was found. The pitch trim position could not be determined conclusively because the trim control cables were broken and disrupted by impact. However, evidence was not sufficient to conclude that full nose-down trim was present at the time of upset. An examination of the autopilot annunciator module indicated that the HDG, NAV, AUTOTRIM and FAIL warning lights may have been illuminated. When the autopilot detects a trim system failure, the trim servo is deactivated and the AUTOTRIM and FAIL warning lights will flash. The AUTOTRIM light will also flash if the trim master switch is turned off, making the autopilot system evidence inconclusive. Because the autopilot system evidence was inconclusive, the role of the autopilot in the accident could not be determined. However, airplane descent information provided by witnesses and impact evidence were consistent with an aerodynamic stall at low altitude and were not consistent with an uncorrected autopilot induced pitch-down excursion.