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US transport safety officials investigating a mid-air emergency on a Boeing 737 Max 9 plane have released thousands of pages of documents, including testimony describing the "chaos" in the moments after the blowout of an unused door.

It came as a two-day National Transportation Safety Board (NTSB) hearing about the 5 January incident on an Alaska Airlines flight got underway.

During the event, Boeing told investigators it will introduce design changes to prevent similar incidents from happening in the future.

The blowout triggered the US aviation giant's second major crisis in recent years.

In the more than 3,000 pages of , the plane's crew described the violent decompression that resulted from the panel detaching mid-flight.

The plane's co-pilot told the investigation there was a "loud bang, ears popping, my head got pushed up into the [head-up display] and my headset got pushed, not off my head, but up almost off my head."

"It was chaos," they said.

"And then, just all of a sudden, there was just a really loud bang and lots of whooshing air, like the door burst open," a flight attendant said.

"Masks came down, I saw the galley curtain get sucked towards the cabin."

The names of the air crew have been redacted in the documents.

At the hearing, Boeing executives were grilled about the manufacture of the aircraft involved in the incident and the lack of paperwork explaining who carried out work on the door plug before the blowout.

A detailed how, after a repair at a Boeing facility, the panel had four bolts missing, which should have helped keep it in place.

"The safety culture needs a lot of work," said NTSB Chair Jennifer ³ÉÈË¿ìÊÖndy, adding that the plane maker needs to take steps to address the issues.

"They are working on some design changes that will allow the door plug to not be closed if there's any issue until it's firmly secured," said Boeing's senior vice president for quality Elizabeth Lund.

The NTSB and Boeing have yet to find out who was responsible for removing and reinstalling the door plug.

But Ms Lund said two workers who are likely to have been involved are now on paid administrative leave.

The incident was the latest major blow to Boeing's reputation.

It resulted in the grounding of Max 9 planes around the world for two weeks, a ban on increasing production, a Federal Bureau of Investigation probe and a management shakeup.

The company recently said it would plead guilty to a fraud charge related to fatal crashes of two of its 737 Max planes more than five years ago.

Last week, Boeing said it had lost $1.4bn (£1.1bn) between April and June.

It has also named aerospace industry veteran and engineer Robert K 'Kelly' Ortberg as its next chief executive.