The Federal Aviation Administration ordered a nationwide ground stop Friday for all airports due to a communication issue affecting air traffic control -- a rare move that thrust the nation's air travel into chaos.

In the hours after the ground stop was issued, more than 2,000 flights were canceled and more than 10,000 were delayed as of 6:20 p.m., according to FlightAware, and lawmakers demanded answers about what had happened.

The FAA's alert said the stop was "due to extraordinary circumstances and for reasons of safety."

Department of Homeland Security Secretary Markwayne Mullin sought to reassure the American public, telling reporters Friday evening that there was no indication yet that the incident was a "nefarious attack" and that everyone is safe.

"This isn't another 9/11. I cannot repeat that enough," he added.

The nationwide ground stop was only the second since 9/11. The last time was in 2023, when the FAA NOTAM system went down and pilots could not get their pre-departure paperwork.

Mullin said the government will look into the root of the problem, but the problem appeared to be "just a telecommunications outage as of now," he told reporters.

When asked by a reporter if it could be a cyberattack, he said they were not ruling anything out.

"As we investigate what caused this, you always got to assume that, as many cyber attacks that take place, that you make that assumption," Mullin said. "But as of right now, that's not known."

Shortly after the stop was issued, Transportation Secretary Sean Duffy said in an X statement that it was "due a communication issue impacting all Air Route Traffic Control Centers."

"FAA technicians are investigating the issue," he said in an X post.

Air Route Traffic Control Centers are responsible for all flights above 18,000 feet. There are 22 of those centers in the U.S.

Canada's civil air navigation service, NAV Canada, told ABC News it is monitoring the situation with the FAA ground stop and there is no impact to Canadian systems.

NAV Canada said it "continues to support the safe movement of air traffic within our airspace, including any aircraft that may be diverted."

Pilots who are in the air have flight plans and are prepared to continue with their flight path in case of such an emergency, according to ABC News contributor John Nance.

"All pilots in that environment know what to do," Nance told ABC News on Friday.

"It is a matter of predictability. Air Traffic Control knows that airplane is going to come out of the sky at some point of time looking for a pathway," he later added.

Mullin told reporters that the federal government will be looking into all possibilities behind the communications issue. When asked by a reporter if it could be a cyber attack, he said they were not ruling anything out.

"As we investigate what caused this, you always got to assume that, as many cyber attacks that take place, that you make that assumption," Mullin said. "But as of right now, that's not known."

AirSpace Intel, the contractor behind the SMART AI system that has been rolling out at facilities nationwide said in a statement on X that "The current ground stop is not based on or a result of the SMART system, any product of ASI or our integration with FAA and ATC."

Mullin said "we're looking at that" when he was asked if there was a link between the rollout and the ground stop.

According to Flightradar24, there were 3,100 airborne aircraft bound for U.S. airports earlier amid nationwide ground stop. No additional aircraft are taking off at this time. Flights that are currently in the air will be diverted to other airports.

Several congressional leaders spoke out about the incident on Friday evening, calling for answers and accountability.

Sen. Tammy Duckworth, an Illinois Democrat, noted in an X post that the "safety of our skies is built on redundancies."

"Whatever led to the nationwide ground stop will warrant a thorough and complete investigation," she wrote. "As Aviation Subcommittee Ranking Member, I'll be demanding answers from the FAA."

Senate Transportation Committee Ranking Member Maria Cantwell said in a statement that her team is in contact with the FAA about the ground stoppage.

"Technology issues related to how planes communicate through primary frequencies, backup frequencies, and other issues will be something the Commerce Committee will be reviewing," the Washington Democrat said.

Earlier in the afternoon, the FAA issued a ground stop at both Chicago's O'Hare and Midway airports due to equipment outage.

ABC News' Victoria Beaule contributed to this report.

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