Four bolts that hold a door plug in place were missing from the Boeing (BA) 737 Max 9 that los✨t paꦬrt of the aircraft in midflight last month, a U.S. regulator has found.

Investigators from the National Transportation Safety Board said in a preliminary report on Tuesday that evidence shows the aircraft, operated by Alaska Airlines (ALK), left the factory missing the bolts.

The mid-exit door plug, which was to be secured in place by four bolts, was manufactured by Spirit AeroSystems (SPR), the NTSB said. Records also show that there were damaged rivets on the doඣor plug, the NTSB report said.

Intervi𒉰ews of Boeing and Spirit AeroSystems’ personnel will be scheduled at a future date, NTSB said.𒊎

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

Boeing said it is " is taking immediate action to strengthen quality."

Shares in Boeing, which suspended its forward guidance amid safety concerns, were little changed Wednesday morning. They are down 17% this year.