The increase in cases was disclosed Wednesday during a National Transportation Safety Board hearing into the accident, which happened during an Alaska Airlines flight on Jan. 5.

Brian Knaup, who helps manage the FAA’s oversight of Boeing, said one of the open cases involves the removal of parts that have already been installed on airplanes in production.

That is apparently what caused the mistake that led to the Alaska Airlines accident: Bolts that were removed to open the door plug for maintenance workers were not replaced when the panel was closed and the plane left a Boeing factory near Seattle.

Knaup’s comment came near the end of a two-day hearing that included discussion of Boeing’s poor tracking of parts-removal jobs. The company failed to document who opened the door plug, and the missing bolts were never found.

Another FAA official overseeing Boeing, Bryan Kilgroe, said he is kept awake at night wondering “especially considering all that has happened since Jan. 5, is why is it so difficult to sustain a corrective action for the long term?”

Boeing said it had no comment.

The safety board released released testimony by Boeing employees who said they were pressured to build planes too quickly and not raise safety concerns.

NTSB Chair Jennifer Homendy cited two employees who worked on aircraft doors where the Alaska Airlines plane was assembled and claimed they were moved to other areas — “Boeing prison” and “a cage” — after the door-plug blowout.

“What sort of impression does that give your employees if you sideline them …? It is retaliation,” Homendy said. She said “sidelining” the two workers runs against Boeing’s policy, which is not to retaliate against workers for unintentional mistakes.

Homendy said the NTSB will survey workers at Boeing’s factory in Renton, Washington, where the Alaska Airlines plane was produced, about the company’s safety culture.

Representatives from Boeing and key supplier Spirit AeroSystems described their “safety management systems,” which encourage employees to voluntarily report safety concerns without fear of punishment. Boeing officials touted their “Speak Up” program for reporting concerns about quality and safety.

However, the president of the machinists’ union local said Boeing often ignores safety concerns raised by the union until he lodges a complaint with federal regulators.

“It really sounds great,” the official, Lloyd Catlin, said of Boeing’s safety plan. “In action on the factory floor, it is not.”

The FAA has been roundly criticized for lax regulation of Boeing ever since two deadly Max crashes in 2018 and 2019 killed 346 people. Those charges gained new momentum after the Alaska Airlines accident.

The agency’s new chief, Mike Whitaker, told Congress in June that FAA oversight “was too hands-off” but is improving. Knaup, a California-based FAA manager, said inspections have increased since the blowout.

FAA safety inspectors “can talk to anyone that’s on the (Boeing factory) floor at any time when they are doing an audit, and we do that,” he told the NTSB.

Door plugs are installed on some 737s to seal a cutout left for an extra exit that was not required on the Alaska jet. The plug on the Alaska plane was opened at a Boeing factory to let workers fix damaged rivets, but bolts that help secure the panel were not replaced when the plug was closed.

The accident on Alaska Airlines flight 1282 occurred minutes after takeoff from Portland, Oregon, on Jan. 5. The blowout left a hole in the plane, oxygen masks dropped and the cockpit door flew open. Miraculously there were no major injuries, and pilots were able to return to Portland and land the plane safely.

A Boeing official said Tuesday that the company is redesigning door plugs so they cannot be closed until they are properly secured. Elizabeth Lund, who was named Boeing’s senior vice president of quality shortly after the blowout, said the company hopes to complete the fix within about a year, and that 737s already in service will be retrofitted.