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“Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening,” Chicago-based United said.

Alaska said that as it began examining its Max 9s, “Initial reports from our technicians indicate some loose hardware was visible on some aircraft.”

The are ratcheting up pressure on Boeing to address concerns that have grown since the terrifying fuselage blowout Friday night. A plug covering a spot left for an emergency door tore off the plane as it flew 16,000 feet (4,800 meters) above Oregon.

Boeing has called an online meeting for all employees Tuesday to discuss safety.

The company, which has had over the years, pledged to “help address any and all findings” that airlines make during their inspections of Max 9 jets. Boeing has delivered more than 200 to customers around the world, but by the Federal Aviation Administration on Saturday until the door plugs can be inspected and, if necessary, fixed.

The door plugs are inserted where emergency exit doors would be located on Max 9s with more than about 200 seats. Alaska and United have fewer seats in their Max 9s, so they replace heavy doors with the plugs.

The panels can be opened for maintenance work. The bolts prevent the mechanism from moving upward on rollers when the plane is in flight.

During Alaska Airlines flight 1282 on Friday night, roller guides at the top of one of the plugs broke — for reasons the investigators don't fully understand yet — allowing the entire panel to swing upward and lose contact with 12 “stop pads” that keep the panel attached to the door frame on the plane, NTSB officials said at a news briefing in Portland.

NTSB Chair Jennifer Homendy said the safety board was investigating whether four bolts that help prevent the panel from sliding up on rollers were missing when the plane took off from Portland or whether they blew off “during the violent, explosive decompression event.”

The interior of the plane suffered extensive damage, but pilots were able to return to Portland and land safely. Officials say there were no serious injuries among the 171 passengers and six crew members.

The lost door panel was found Sunday near Portland in the back yard of a school teacher's home. NTSB officials said it will be sent to the agency's lab in Washington, D.C., for detailed study that might help pinpoint why the plug broke loose.

Alaska and United have canceled hundreds of flights since the weekend because of their grounded planes. Alaska has 65 Max 9s, and United has 79. The airlines waited until Monday before Boeing and the FAA completed instructions for how to inspect their planes.

The jet involved in Friday’s blowout is brand-new, having been put in service in November. After a cabin-pressurization system warning light came on during three flights, the airline stopped flying it over the Pacific to Hawaii. Some aviation experts questioned why Alaska continued using the plane on overland routes until it figured out what was causing the pressurization warnings.

Homendy said Monday, however, that NTSB has seen no evidence to link the warnings with the blowout of the door plug.

The Max is the newest version of Boeing’s 737, a twin-engine, single-aisle plane that debuted in the late 1960s and has been updated many times. The 737 has long been a workhorse for airlines on U.S. domestic routes.

Shares of Boeing fell 8% and Spirit AeroSystems, which installs the door plugs on Max jets, dropped 11% on Monday.

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Koenig reported from Dallas and Krisher reported from Detroit. Associated Press reporter Terry Spencer in Fort Lauderdale, Florida, contributed to this report.