President Trump was informed within 30 minutes after the Federal Aviation Administration imposed a nationwide ground stop Friday afternoon, according to U.S. officials.

The ground stop resulted in thousands of canceled flights and stranded travelers across the U.S. The FAA announced that all planes would be grounded at 3:36 p.m. ET — while 3,500 flights were in the air.

Within minutes, the FBI, director of national intelligence, Department of Homeland Security, Transportation Department, Justice Department, Defense Department, Joint Chiefs of Staff, U.S. Cyber Command, FAA and Federal Emergency Management Agency began looping each other in.

Transportation Secretary Sean Duffy said in a statement at 4:16 p.m. ET that the ground stop was necessary due to “a communication issue impacting all Air Route Traffic Control Centers,” and that FAA technicians were investigating. Within minutes, the White House led a call with the agencies to determine what had happened. A game plan was put in motion.

The intelligence community soon preliminarily ruled out the likelihood of nefarious activity after searching through its various channels for chatter or pings. Nothing came up.

Senior staff briefed Mr. Trump, who was in an economic policy meeting, one U.S. official said. The staff relayed his directives to others on the multi-team call, led by White House chief of staff Stephen Miller.

Later Mr. Trump met with Duffy and DHS Secretary Markwayne Mullin.

Mr. Trump started calling congressional leaders on Friday to work on getting the FAA more funding. The White House congressional team called key congressional members to work with them on air traffic controller modernization.

The Homeland Security Council invited key members of the government and some outside stakeholders to a meeting Monday to discuss the next steps and what should be done to prevent a recurrence as modernization efforts continue. 

The council is an interagency government body responsible for coordinating national homeland security policy. Established by President George W. Bush in 2001, under the current White House structure, the Council operates through the National Security Council framework and can draw on senior officials from agencies like the Transportation Department, FAA, DHS and Defense Department.

But not all planes were grounded during the four-hour stop: medical services flights were green-lighted to proceed to their intended destinations.

At a news conference on Saturday, Duffy said the trouble apparently began as FAA telecommunications contractor L3Harris Technologies performed maintenance updates at a remote facility, replacing aging copper communications lines with fiber optic connections. During that work, a router began generating more than 1,000 connection attempts every four to five seconds, overwhelming routers across the entire FAA network.

Mullin and Duffy were more than 30 minutes late for a 7:45 p.m. ET press conference on the ground stop Friday because they were close to getting the grounding lifted and didn’t want to interrupt the effort to hold the presser, U.S. officials said.

While federal officials have identified a likely sequence of events behind Friday’s disruption, key questions still remain about the underlying cause of the systemwide failure.