At about 1825 Pacific time, a Cessna 172K struck trees while attempting to land at Nevada County Air Park. The pilot and passenger were killed. The flight originated as an IFR flight from Grass Valley to Palo Alto, but as the airplane approached the destination the transponder code changed to indicate a communications failure. Bay TRACON determined the pilot could receive but not transmit. Because of the Class B communications requirement, Bay TRACON suggested the pilot try to contact Stockton Tower. The pilot contacted Stockton while 12 miles west of the airport, and Stockton could barely hear his transmissions. The pilot advised Stockton to cancel his IFR flight plan and he would proceed VFR back to Grass Valley. Witnesses observed the pilot make two attempts to land at Grass Valley. According to airport personnel, the airport had been zero visibility and zero ceiling in fog for 20 minutes prior to his arrival.