Air crash investigators have said it could take years to determine exact cause of why Asiana Airlines flight crash-landed at San Francisco's airport on Saturday. Deborah Hersman, head of USA's National Transportation Safety Board, said it was too early to say whether or not the accident was result of pilot error. Many analysts have already concluded that it was very unlikely that mechanical error was involved, citing excellent safety record of Boeing 777 aircraft. Kevin Hiatt, CEO of Flight Safety Foundation, told reporters that any conclusions about what caused crash are months, if not years, away. What is known is that plane was flying too slowly as it approached runway. As firefighters extinguished last of flames from burning wreckage, survivors described horror of escaping from aircraft. They said it was miracle that 305 of 307 people on board made it out alive. Two Chinese students died in accident. Passengers were quick to praise professionalism, quick thinking and selflessness of flight attendants, one of whom hauled injured passenger to safety. Frequent Asiana flyer Eugene Rah recounted heroism of flight attendant Jiyeon Kim, saying: "She was hero. This tiny, little girl was carrying people piggyback, running everywhere, with tears running down her face. She was crying, but she was still so calm and helping people."