Homeland Security Secretary Markwayne Mullin now says the failure behind the nationwide ground stop looks mechanical and that there is no indication of a cyber attack. Earlier he had said officials were not ruling out any cause.
"It appears this was mechanically driven."
He made the remark at the Friday night briefing, Scripps News reports.
CBS News reports Mullin repeating that there was no cyber attack. He also said the stop was not related to Thursday's global Microsoft outage.
According to Gizmodo, Mullin said new equipment and technology were overwhelming the older system. Gizmodo's account of his wording is partly paraphrased.
Separately, a congressional official told Scripps News that neither the White House and National Security Council nor the Department of Defense knew the cause while the incident was under way.
Mullin's position is firmer than before, but he has still not formally ruled out an attack. His explanation that the failure appears mechanical has not been independently checked.