US safety officials say Boeing door-panel missing bolts

US transport investigators say the door panel that blew off a Boeing jet last month was missing bolts.

The National Transportation Safety Board said in a preliminary report released on Tuesday that the plane had been repaired at a Boeing factory from August to September last year. The investigators say photos from that time and other evidence show the panel lacked four bolts.

The Boeing 737 Max 9 probe began after it was forced to make an emergency landing on an Alaska Airlines flight. There were no serious injuries in the January 5 incident in the western state of Oregon.

Boeing CEO David Calhoun said in a statement, "Whatever final conclusions are reached, Boeing is accountable for what happened. An event like this must not happen on an airplane that leaves our factory."

He added that Boeing plans to strengthen quality and stakeholder confidence.