US transport officials are looking into the cause of a major flight disruption, with 16,000 flights canceled in the past 24 hours. The largest grounding since the 9/11 attacks has left many travelers stranded. Officials have not ruled out incompetence or faulty equipment as the cause.

The scale of the disruption has raised concerns about the reliability of air travel in the US. No definitive explanation has been given, and investigations are ongoing.