(NewsNation) — The National Transportation Safety Board says investigators still don’t know who worked on a Boeing 737 Max 9 door plug that blew off during a January midflight emergency.

“The absence of those records will complicate the NTSB’s investigation moving forward,” NTSB Chair Jennifer Homendy wrote in a letter to a Senate committee that’s looking into the Jan. 5 accident the plane operated by Alaska Airlines. “We still do not know who performed the work to open, reinstall, and close the door plug on the accident aircraft.”

Homendy added that investigators sought security camera footage that might help identify who worked on the panel in September, but they were told the video was overwritten after 30 days — months before the blowout.

In a statement to NewsNation, Boeing said its standard company practice is to erase video after 30 days.

The NTSB previously reported that four key bolts were missing from the door plug that blew out on the plane, and investigators are still trying to determine why.

The agency clarified its intent is not to penalize the employees but to understand Boeing’s quality assurance process.

Homendy’s latest letter to the Senate Commerce Committee was a follow-up to her appearance before the panel last week. Shortly after her testimony ended, Boeing provided names of 25 employees who work on doors at the company’s 737 factory near Seattle.

She said, however, the company still hasn’t said which of the workers removed the panel, which plugs a hole left when extra emergency doors are not required on a plane. She said she even called Boeing CEO David Calhoun.

“He stated he was unable to provide that information and maintained that Boeing has no records of the work being performed,” Homendy wrote. Boeing did not comment on the phone call.

Boeing issued a brief statement vowing, as it has many times, to support the investigation.

“We will continue supporting this investigation in the transparent and proactive fashion we have supported all regulatory inquiries into this accident,” the company said in a statement.

The Department of Justice has launched a criminal investigation into the incident.

Additionally, the New York Times reported that an audit by the FAA uncovered “dozens of issues” with Boeing’s 737 Max production line. As a result, Southwest Airlines, which exclusively uses these planes, lowered its growth projection for 2024.

In another development, police confirmed this week that a Boeing whistleblower was found dead just one day after providing testimony in his deposition against the company as part of a civil lawsuit. The coroner’s office reported the cause of death was an apparent self-inflicted gunshot wound.

The Associated Press contributed to this report.