WASHINGTON (Reuters) -The outgoing U.S. transportation secretary said on Monday that Boeing has more to do and its efforts to improve its culture are unfinished after a door panel missing four key bolts flew off a new 737 MAX 9 in mid-air in January 2024.

"There were real deficiencies ... there is much more to do," Pete Buttigieg told Reuters in an interview. "I think the culture change at Boeing is something that is a real work in progress and the only way to fully assess it will be to see they can consistently improve results."

Boeing declined to comment.

Last week, the outgoing head of the Federal Aviation Administration said tougher oversight of Boeing would continue indefinitely after the incident in which a door panel missing four key bolts flew off the new 737 MAX Alaska Airlines plane at 16,000 feet.

The Jan. 5, 2024 incident prompted FAA Administrator Mike Whitaker to cap production at 38 737 MAX planes per month and temporarily ground 170 airplanes. The incident exposed serious safety issues at the U.S. planemaker and contributed to the departure of its then-CEO Dave Calhoun.

Whitaker last February ordered Boeing to implement a safety and quality improvement plan and previously acknowledged prior oversight "was too hands off." Whitaker has said it could take five years for Boeing to fully fix its culture.

Boeing on Friday released an update on its safety and quality efforts, saying it has instituted new random quality audits and significantly reduced defects in 737 fuselage assembly at supplier Spirit AeroSystems by increasing inspection points and implementing a customer quality approval process.

The FAA announced a new audit of Boeing in October.

Buttigieg, who repeatedly sparred with air carriers, said separately that he hopes the incoming administration of President-elect Donald Trump maintains his department's efforts to ensure consumer protections for airline passengers in the event of flight delays.

"I certainly didn't see them campaigning on a platform of being less pro-passenger than we are," Buttigieg said.

Buttigieg, who unsuccessfully ran for president in 2020, has said he has not made any plans for future jobs, but is viewed by some Democrats as a potential candidate for Michigan governor in 2026.