Transportation Secretary Sean Duffy confirmed that an investigation is underway into whether contractor L3Harris or faulty equipment caused the problems with the FAA system upgrade that led to Friday's nationwide ground stop. According to Duffy's preliminary account, an L3Harris contractor was doing routine maintenance at a remote FAA location, switching old copper wiring to fiber while replacing a router. That router then began flooding other routers in the air traffic control system, overloading it. Investigators are still working to determine whether it was installer error or a faulty router. L3Harris says it is supporting the investigation, and officials say they have found no evidence of a cyberattack.

Asked why the work was done at 3 p.m. on a Friday instead of overnight on a Saturday, when fewer planes are in the air, Duffy said the upgrade took place at a remote location, where such work is normally done during the day. Larger facilities switch over at night, he added, and the timing of this kind of work will be reevaluated. The outage halted flights across the country for hours and left thousands of flights canceled, so the question of timing is likely to get a lot of scrutiny. Duffy has also pointed to the age of the system and urged Congress to fund a major overhaul. Until the investigation finishes, it remains unclear who or what is responsible.