As of early Thursday, just over 440 flights in, to and out of the U.S. were delayed, according to flight tracking website FlightAware.

According to preliminary investigation findings released by NTSB investigators on Tuesday, four key bolts were "missing" when a door plug blew off an Alaska Airlines 737 Max 9 in midair last month.

A door panel that flew off a Boeing 737 MAX 9 jet mid-flight on Jan. 5 appeared to be missing four key bolts, according to an initial report by a U.S. safety board investigating the incident.