Nearly a month after the National Transportation Safety Board determined that four bolts intended to secure a panel that blew off an Alaska Airlines plane during a January 5 flight were missing, the agency is no closer to questioning any of the 25 door crew employees involved.

NTSB chair Jennifer Homendy told Congress in a letter that the bolts were removed and not replaced at Boeing’s factory in Washington. The door plug was opened so that rivet repair work could be performed.

“To date, we still do not know who performed the work to open, reinstall, and close the door plug on the accident aircraft,” Homendy stated. “Boeing has informed us that they are unable to find the records documenting this work. 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 emphasized. NTSB wants to question these employees about Boeing’s quality-assurance processes and safety culture.

While Boeing has named all employees who reported to the door crew manager in September 2023, it did not identify which personnel conducted the door plug work. CEO David Calhoun maintains that Boeing has no records of the work being performed.

The catastrophic failure occurred January 5 when a door plug blew off a new Alaska Airlines Boeing 737 Max 9 at 16,000 feet, imperiling 171 passengers and 6 crew members.

In the fallout of the accident, Boeing has promised immediate action to strengthen quality. In addition, the U.S. aerospace giant said it will open its factory to 737 customers to conduct their own additional reviews.

The National Transportation Safety Board will hold a hearing Aug. 6-7 on its investigation into how and why a door plug departed from a Boeing 737-9 MAX passenger jet during flight.