PORTLAND, Ore. (AP) — Federal investigators are learning more about how a door panel flew off an Alaska Airlines jetliner last week. National Transportation Safety Board officials said Monday that the door panel slid up before flying off the plane. And they’re looking into whether four bolts that were supposed to help hold the panel in place might have been missing when the plane took off. Those comments came after Alaska Airlines and United Airlines reported finding loose parts in the panels — or door plugs — of some other Boeing 737 Max 9 jets. The findings are putting more pressure on Boeing to address safety concerns. The aircraft maker has called an online meeting for all employees Tuesday to talk about safety.