Homeland Security Secretary Markwayne Mullin says President Donald Trump has been briefed on the nationwide ground stop and has spoken by phone with Transportation Secretary Sean Duffy, and that officials are "not ruling out anything at this point", including a cyber attack.
CNN reports Mullin telling reporters at the White House: "There is no indication that we've had, no reason to believe, that as of right now this was a threat to the United States." He said he would not rule out any cause for the outage, including a cyber attack.
On the president's involvement, Mullin said of Duffy: "He was on the phone earlier with the president and several of us. And they're doing a phenomenal job in a crisis mode."
NBC News reports Mullin saying those currently on flights have no reason for concern.
This is consistent with his earlier remarks, which were qualified with "as of yet" and "as of right now". The difference is that he has now said in terms that no cause is excluded. Neither the FAA nor Duffy has yet given a cause beyond a communications issue.