Americans have been told that there is no terrorist attack unfolding, but the US government isn't ruling out any potential causes following the communication issue and ground stop.
He said: "We're getting a tonne of calls saying, 'Is this a terrorist attack?' Everyone's safe. There's no need to be concerned about your loved ones. As we investigate what caused this, you've always got to assume, with so many cyber attacks that take place, but that's not known. First thing, we're going to restore communications then get normal operations."
Mullin added there was "no reason" to connect this to the Microsoft outage, and said: "This is not a safety [issue] to the American people and not a 9/11."
The US Federal Aviation Administration (FAA) has grounded all flights nationwide because of communications problems at regional air traffic control centres.
The FAA notice said the ground stop was being imposed “due to extraordinary circumstances and for reasons of safety”.
Transportation secretary Sean Duffy said there is a communications issue affecting the 22 regional air traffic control centres that direct planes en route between airports nationwide.
He said FAA technicians are investigating.
This is the first nationwide ground stop since January 2023 when a key FAA computer system malfunctioned.
Before that the last time had been after the September 11 2001 attacks.
Secretary Mullin said there was no indication yet that “this was a nefarious attack against us. It’s just a telecommunications outage”.
“Everybody’s safe. There’s no reason to be concerned about your loved ones if they’re flying,” he said.
There is no timeline for restarting flights.
Mr Mullin said investigators will look at whether there is any link to the FAA’s nationwide rollout of a new artificial intelligence system to help identify problems and reroute planes.
But that system operates out of one location near Washington DC, and just sends recommendations to the air traffic control centres.
The government has been working to overhaul the outdated technology that air traffic controllers rely on since Congress approved 12.5 billion dollars for the project last year.
Miles of old copper wires have been replaced with fibre optic lines and old radio systems are gradually being replaced at airports across the country.
But much work remains.
Mr Duffy has said that another 30 billion dollars will be needed to complete the work, and the White House has begun lobbying Congress to approve it.
The problem appeared similar to a regional air traffic control communication outage in September that snarled hundreds of flights across the north east, aviation safety expert Steve Arroyo said, except this time on a nationwide scale.
It may be linked to the outdated technology used by air traffic controllers.
“If you don’t have that ability, whether it’s because the old wires that they have are just run down or for whatever reason, you’ve got to stop airplanes on the ground,” said Mr Arroyo, who is a former United Airlines pilot.
“And not put them in the air where you’re going to risk not being able to communicate with them.”
Friday’s ground stop was different to the one after September 11 because planes were not being told to land immediately and pilots probably were able to communicate with ground controllers at airports, Mr Arroyo said.