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

The NTSB’s 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,” she said.

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

Last week, Homendy said she spoke to Boeing’s 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 did not immediately comment.

The justice department has opened a criminal investigation into the midair emergency.

On Friday, Boeing said it believes required documents detailing the removal of the door plug during production were never created.

Boeing said its working hypothesis was that “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 grounded the Max 9 model for several weeks in January, barred Boeing from increasing the Max production rate and ordered the aircraft maker to develop a comprehensive plan to address “systemic quality-control issues” within 90 days.