Investigators have concluded that the explosion and fire that killed a 24-year-old Greenfield man at an apartment complex was caused by a buildup of gasoline fumes near a running lawnmower. Andrew J. Knutson died in the explosion and fire in a maintenance shed at the Willowick Apartments, 7020 W. Southridge Drive, shortly before 10 a.m. June 22. When police and firefighters arrived, they found Knutson lying outside the shed that was fully engulfed in flames, Greenfield police said.