The White House quickly squashed speculation of a potential cyber attack. President Biden's Transportation Secretary, Pete Buttigieg, has since ordered an investigation of NOTAM's outage. 

Bloomberg reports that a person familiar with the FAA investigation might already know what caused headaches for tens of thousands of travelers -- just weeks after the Southwest Airlines debacle. 

The person said unspecified "personnel" corrupted a file on the NOTAM system, resulting in a nationwide outage.

The preliminary indications are that two people working for a contractor introduced errors into the core data used on the system known as Notice to Air Missions, or Notam, according to a person familiar with the FAA review. The person asked not to be identified speaking about the sensitive, ongoing issue. 

Like other computer systems that are critical to operating flights, the FAA has imposed procedures to ensure data aren't damaged by technicians working on them, said the person. The file or files were altered in spite of rules that prohibit those kind of changes on a live system. --Bloomberg