The pilot who crashed into a crowded Concord shopping mall two days before Christmas in 1985 was flying too low and became disoriented in a cloud bank, the National Transportation Safety Board said. The determination that pilot James Graham was at fault ends nearly two years of investigation into the Dec. 23 crash that killed Graham, his two passengers and four Christmas shoppers. Another 78 people were injured, 17 seriously. It also ends speculation that mechanical failure caused the crash. The board ruled that no mechanical failure was involved. Nearly 50 lawsuits are pending against Graham's estate, the plane manufacturers, Sunvalley Mall, the city of Concord and Contra Costa County. Attorneys are legally barred from introducing the report in court, but one of the lawyers, Gerald Sterns, said it will help the victims' cases indirectly by steering independent investigators toward the evidence.