US investigators say video footage of door panel that flew off Boeing jet mid-flight was overwritten

The head of the National Transportation Safety Board (NTSB) said on Wednesday investigators still do not know who worked on a Boeing 737 MAX 9 door plug involved in a Jan. 5 Alaska Airlines midair emergency and that video footage was overwritten.

NTSB Chair Jennifer Homendy said in a letter to senators that investigators sought security camera footage when the door plug was opened and closed in September but were informed the material was overwritten.

"The absence of those records will complicate the NTSB’s investigation moving forward," Homendy said. "To date, we still do not know who performed the work to open, reinstall, and close the door plug on the accident aircraft."

The NTSB said previously four key bolts were missing from the door plug that blew out on the plane.

Last week, Homendy said she spoke to Boeing (BA.N), opens new tab CEO David Calhoun "and asked for the names of the people who performed the work. He stated he was unable to provide that information and maintained that Boeing has no records of the work being performed."

Boeing said it "will continue supporting this investigation in the transparent and proactive fashion we have supported all regulatory inquiries into this accident. We have worked hard to honor the rules about the release of investigative information."

A Boeing official, speaking on condition of anonymity, told Reuters the planemaker standard practice is to overwrite security videos after 30 days but declined to answer additional questions.

The Justice Department has opened a criminal investigation into the midair emergency.

On Friday, Boeing said it believes required documents detailing the door plug removal were never created, adding its working hypothesis was "the documents required by our processes were not created when the door plug was opened."

Homendy last week criticized what she called Boeing's lack of cooperation and failure to disclose some documents, including on the door plug opening and closing, as well as the names of 25 workers on the door crew at the 737 factory in Renton, Washington. After Homendy's comments, Boeing provided the 25 names.

In the aftermath of the incident the Federal Aviation Administration (FAA) grounded the MAX 9 for several weeks, barred Boeing from increasing MAX production and ordered the company to address "systemic quality-control issues" within 90 days after an audit found fault with Boeing's manufacturing processes.

Separately, the NTSB wrote Boeing on Wednesday reminding them they face restrictions on information they can release publicly as a party to the investigation.

"Releasing investigative information without context is misleading to Congress and the public and undermines both the investigation and the integrity of the NTSB," the agency wrote.