Security camera footage showing work being done on a Boeing Max 9 door plug that later blew out mid-air has been overwritten, the head of the National Transportation Safety Board said.

In a letter to the Senate Committee on Commerce, Science, and Transportation that is reviewing the incident and Boeing’s role in it, NTSB Chair Jennifer Homendy said her agency still lacks critical information about the chain of events that led up to the January incident that caused an Alaska Airlines flight carrying 177 people to make an emergency landing.

‘We still do not know who performed the work to open, reinstall, and close the door plug on the accident aircraft,’ Homendy wrote. ‘Boeing has informed us that they are unable to find the records documenting this work.’

She continued: ‘A verbal request was made by our investigators for security camera footage to help obtain this information; however, they were informed the footage was overwritten. The absence of those records will complicate the NTSB’s investigation moving forward.’

Homendy said in the letter that she also personally called Boeing CEO Dave Calhoun and asked him for the names of the workers who performed the work in question. But, Homendy said, Calhoun stated he was ‘unable to provide that information and maintained that Boeing has no records of the work being performed.’

A Boeing spokesperson said the company, like many others, does not retain security footage for longer than 30 days. The Alaska Airlines plane in question was in the factory last year in September and delivered in October.

“We will continue supporting this investigation in the transparent and proactive fashion we have supported all regulatory inquiries into this accident,’ Boeing said. ‘We have worked hard to honor the rules about the release of investigative information in an environment of intense interest from our employees, customers, and other stakeholders, and we will continue our efforts to do so.” 

Homendy had previously publicly reprimanded Boeing for failing to turn over information her agency had requested, calling it “absurd.”

Last week, NBC News confirmed a Wall Street Journal report that the U.S. Department of Justice had opened a criminal case into the incident.

On Monday, South Carolina officials confirmed a Boeing whistleblower was found dead of an apparent self-inflected gunshot wound. The former employee, John Barnett, 62, was preparing to testify in a deposition in a federal legal action against Boeing dating back to at least 2017.

Barnett’s family in a statement that he had encountered “a culture of concealment” that valued “profits over safety” at Boeing.

The New York Times also reported this week that an FAA audit of Boeing’s 737 Max production reportedly found ‘dozens of issues.’

This week, major carriers that fly Boeing fuselages including Alaska Airlines, Southwest, and United Airlines said they may have to trim capacity and push back orders as a result of Boeing’s issues.

“Boeing needs to become a better company and the deliveries will follow that,” Southwest Airlines CEO Bob Jordan said at a JPMorgan industry conference Tuesday, according to CNBC.