WASHINGTON (Reuters) -The head of 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 mid-air 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.

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 ( BA ) 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 ( BA ) has no records of the work being performed."

Boeing ( BA ) did not immediately comment.

The Justice Department has opened a criminal investigation into the mid-air emergency.

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

Boeing ( BA ) said 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 ( BA ) 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 ( BA ) provided the 25 names.

In the aftermath of the incident the Federal Aviation Administration (FAA) grounded the MAX 9 for several weeks in January, barred Boeing ( BA ) from increasing the MAX production rate and ordered Boeing ( BA ) to develop a comprehensive plan to address "systemic quality-control issues" within 90 days.

Separately, the NTSB wrote Boeing ( BA ) 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," Homendy wrote.