The FAA has told airlines that the nationwide outages behind the US ground stop came from a problem with network routers, CNN reports, citing an airline source. The FAA has not confirmed this.
CNN, reporting at 6.37pm ET (11.37pm UK), says the problem is believed to have started at Chicago Midway International Airport and cascaded nationwide.
Separately, CBS News, citing multiple sources, reports that an update to the private communications network connecting America's air traffic control system caused it to reboot, leading to a crash and a communications outage. CBS says the system is operated by L3Harris, which did not immediately respond to its request for comment.
These are the first reports of a specific technical cause, and they differ in detail. One describes a router fault and the other a crash triggered by an update. Neither has been confirmed by the FAA or Transportation Secretary Sean Duffy, and neither has been independently checked. An airline source had earlier told CNN the root cause was still unclear.
Clash Report, summarising both accounts, says the affected network handles radio communications between controllers and pilots. Earlier reports of radio frequency outages also remain unconfirmed.