The extended grounding of some Boeing 737 Max jetliners is adding to pressure on Boeing and the subcontractor that made the fuselage and installed a panel that blew out leaving a gaping hole in an Alaska Airlines plane last week. Investigators know the sequence of events that led to the blowout Friday night, but they don't know the cause. A key question is whether bolts used to help secure the panel, called a door plug, were installed. A National Transportation Safety Board investigator says the bolts have not been recovered and the agency won't know if they were even in place until the door plug is examined in a laboratory. Adding to Boeing's problems, Alaska Airlines and United Airlines — the two U.S. carriers that fly the Max 9 — reported finding loose bolts and other hardware in other panels, suggesting quality issues with the door plugs are not limited to one plane. The plugs are installed in Max 9 fuselages by subcontractor Spirit AeroSystems, which was spun off by Boeing in 2005.