WASHINGTON, March 13 Reuters 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 NTSBs 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 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…