Nearly three weeks after a hole blew open on a Boeing 737 Max 9 during an Alaska Airlines flight, terrifying passengers, new details about the jet’s production are intensifying scrutiny of Boeing’s quality-control practices.

About a month before the Max 9 was delivered to Alaska Airlines in October, workers at Boeing’s factory in Renton, Washington, opened and later reinstalled the panel that would blow off the plane’s body, according to a person familiar with the matter.

The employees opened the panel, known as a door plug, because work needed to be done to its rivets — which are often used to join and secure parts on planes — said the person, who asked for anonymity because the person isn’t authorized to speak publicly while the National Transportation Safety Board conducts an investigation.

The request to open the plug came from employees of Spirit AeroSystems, a supplier that makes the body for the 737 Max in Wichita, Kansas. After Boeing employees complied, Spirit employees who are based at Boeing’s Renton factory repaired the rivets. Boeing employees then reinstalled the door.

An internal system that tracks maintenance work at the facility, which assembles 737s, shows the request for maintenance but does not contain information about whether the door plug was inspected after it was replaced, the person said.

The details could begin to answer a crucial question about why the door plug detached from Flight 1282 at 16,000 feet, forcing the pilots to make an emergency landing at Portland International Airport in Oregon minutes after taking off Jan. 5. The door plug is placed where an emergency exit door would be if a jet had more seats. To stay in place, the plug relies primarily on a pair of bolts at the top and another pair at the bottom, as well as metal pins and pads on the sides.

The Seattle Times reported earlier Wednesday that Boeing had removed and reinstalled the door plug.

The Federal Aviation Administration on Wednesday approved detailed instructions for how airlines should inspect the door plugs on about 170 grounded planes. The instructions tell airlines to re-torque fasteners on the door plug, check the plug’s bolts and fittings, and fix any damage they find. Airlines can begin flying the jets again after completing the inspections.

United said it would begin inspecting its 79 Max 9 planes under the new guidelines and planned to start using them again on flights Sunday. Alaska Airlines said it planned to put a “few planes” back into service Friday and expected to complete inspections on all 65 of its Max 9 jets over the next week.

Also Wednesday, Boeing CEO Dave Calhoun met privately with lawmakers in Congress. It was the second time in recent years that the company and its leaders have had to answer for serious problems with its planes. In 2018 and 2019, two crashes of the 737 Max 8 killed 346 people.

“The American flying public and Boeing line workers deserve a culture of leadership at Boeing that puts safety ahead of profits,” Sen. Maria Cantwell, D-Wash., chair of the Commerce, Science and Transportation Committee, said Wednesday. She added that she would hold hearings “to investigate the root causes of these safety lapses.”

How the panel was installed at Boeing’s factory will almost certainly be a focus of federal investigations. In addition to the NTSB, the FAA is looking into the incident and manufacturing practices at Boeing and Spirit.

Citing the open NTSB investigation, Boeing referred questions to the agency, which declined to comment. The FAA did not immediately respond to a request for comment about Boeing’s handling of the door plug. A spokesperson for Spirit AeroSystems said the company remained “focused on the quality of each aircraft structure that leaves our facilities.”

John Cox, a former airline pilot who runs a safety consulting firm, said the new information about the door plug, if it was correct, would be an indication of “process failure” and raise questions about Boeing’s entire manufacturing operation.

“Are there similar issues in other areas besides the door?” he said. “You’ve got to look at the whole assembly process.”

The FAA said Wednesday that it would not allow Boeing to expand production of any new planes in the 737 Max series, a linchpin of the company’s commercial plane business, until the agency was convinced that quality control had improved.

Calhoun suggested this month that a manufacturing lapse had been responsible for the door plug’s blowout. But it hadn’t been clear whether the lapse, which Calhoun referred to as a “quality escape,” occurred at Boeing’s factory in Renton or Spirit’s facility in Wichita, where the door plug was first installed.

The incident has raised fresh concerns about Boeing’s quality control among investors, airline executives, pilots, passengers and others in addition to regulators. Boeing’s share price has fallen 14% since the blowout.

In recent days, several airline executives have sharply criticized the company, a major supplier that they rarely complain about publicly.

“I am angry,” Ben Minicucci, CEO of Alaska Airlines, told NBC News on Tuesday, adding that the airline found loose bolts on “many” of its Max 9s. “My demand on Boeing is what are they going to do to improve their quality programs in-house.”

Scott Kirby, United Airlines’ CEO, told CNBC on Tuesday that “the Max 9 grounding is probably the straw that broke the camel’s back for us.” He also said he was worried that Boeing would not be able to deliver another 737 Max plane the airline had ordered, the Max 10, anytime soon. That model has not yet been certified by the FAA.

“We’re going to at least build a plan that doesn’t have the Max 10 in it,” Kirby said.

For now, Boeing remains in damage-control mode. Calhoun visited the Spirit AeroSystems factory last week — a plant that the plane maker sold in 2005. And Boeing said this week that it was planning to hold a “quality stand-down” Thursday, during which production, delivery and support teams would stop work to attend learning sessions on quality.

The company said it intended to conduct similar pauses at all of its commercial airplane factories and fabrication sites in the coming weeks.

This article originally appeared in The New York Times.

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