 Boeing says it can't find paperwork related to the installation of a door plug that later blew off an Alaska Airlines 737 MAX 9 jet. In a letter, one Boeing executive says it's likely the paperwork never existed. The head of the National Transportation Safety Board says the NTSB still doesn't know who was responsible for failing to attach the door plug properly. The NTSB says four bolts were missing. Elizabeth Lund is a senior vice president at Boeing.