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, Wash., 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, Kan. After Boeing employees complied, Spirit employees who are based at Boeing’s Renton factory repaired the rivets. Boeing employees then reinstalled the door.