Transportation Secretary Sean Duffy has blamed ageing air traffic control equipment and overloaded routers for the nationwide US ground stop. It is the first official explanation of the outage.
Speaking at a White House briefing on Friday evening, he said: "Our routers actually got overloaded with data and… they started to give us inconsistency in the data from those routers. And because of that, we shut the system down."
Duffy said some routers had been replaced during his tenure. The old hardware went down while the new switches worked, he said, according to CNN and NBC News.
His account fits the earlier anonymous reports of a router fault, but he did not confirm that the problem began at Chicago Midway. Axios reports that neither Duffy nor Mullin specified the initial trigger. Duffy said: "We're still working on the details of what went wrong today."
Mullin blamed new equipment overwhelming the old framework, CNN reports. That sits awkwardly with Duffy's statement that the new equipment kept working.
CBS News still cites sources saying a network update caused a crash. Fox Business says the FAA told Fox News that claims of a software outage issue are false.