Missing bolts were the culprit behind the Alaska Airlines blowout, preliminary investigation finds

Bolts that would have held the door in place weren't attached when Boeing delivered the plane, investigators said.

• The National Transportation Safety Board's published a preliminary report on Alaska Airlines' blowout Tuesday.
• The report said several bolts were missing from a door plug that separated mid-flight. 
• Bolts on the door weren't installed to begin with, the report said. 

Several bolts were missing from a door plug that blew out mid-flight from an Alaska Airlines Boeing 737 Max 9 plane in January, a preliminary report from the National Transportation Safety Board said Tuesday.

Bolts that would have held the door in place weren't attached before the plane took off on January 5 — or before it was even delivered by Boeing, investigators said.

The report sought to determine whether the bolts became dislodged during the incident or if they were never there to begin with.

Seven passengers and one flight attendant sustained minor injuries during the incident.

A full NTSB report is still in the works.

This is a developing story. Check back for more information.