Investigation into why a panel blew off a Boeing Max 9 jet focuses on missing bolts

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. At 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. Spirit has a history of manufacturing problems, many uncovered in a U.S. House probe of two fatal crashes involving Boeing 737 Max 8 planes.