The White House said earlier that there is “no evidence of a cyberattack” at this point in the investigation into the cause of the issue.

During an interview with CNN, U.S. Transportation Secretary Pete Buttigieg confirmed there is no indication of a cyber attack, but added that the U.S. is not ruling it out at this time.

Buttigieg said he directed an “after-action process” to determine root causes and recommend next steps.

“A fare difference waiver for all Delta and Delta Connection flights on Wednesday, Jan. 11, will give customers additional flexibility to change their flights, even if their flight isn’t delayed or canceled,” Delta wrote in a statement Wednesday.

The airline has been forced to cancel 130 flights after “all other options were exhausted,” warning there is a potential for further delays and cancellations.

“Delta will continue to work with the FAA on strategic delay programs at certain airports throughout the day,” the statement continues. “These are necessary to prevent further congestion on the ground and to ensure we have enough available gate space at airports.”

For additional information on Delta’s waiver, click here.

A spokesperson from UPS told Newsweek that flights have resumed and there is “minimal impact” to today’s scheduled deliveries.

“I just spoke to [U.S. Secretary of Transportation Pete] Buttigieg,” Biden said Wednesday at the White House, the Associated Press reported. “They don’t know what the cause is. But I was on the phone with him about 10 minutes. I told him to report directly to me when they find out.”

Biden has directed the DOT to conduct a full investigation into the cause. However the White House said there is “no evidence of a cyberattack at this point.”

Both airports kept the restriction in place after the FAA officially lifted its ground stop earlier this morning. Flights are now beginning to board and prepare for departure, the airport said in separate tweets.

Residual delays or cancellations will likely continue throughout the day as a result of the FAA’s system outage Wednesday morning.

Another day of travel chaos in the U.S as passengers on thousands of flights are delayed. Although the Federal Aviation Administration (FAA) has now lifted the ground stop, the number of delayed flights continues to climb.

More than 6,100 flights within, into, or out of the U.S. have been delayed as of 10:48 a.m. ET, according to FlightAware. More than 1,000 others have been canceled.

Where are the most flights delayed? Atlanta’s Hartsfield-Jackson International (ATL), Chicago O’Hare International (ORD) Denver International (DEN) and Dallas-Fort Worth International Airports (DFW) are showing the highest amount of flight delays at this time.

Today’s problems follow a series of issues over the Christmas holiday when weather caused days of cancellations and delays. Southwest Airlines also canceled more than half of its flights for a couple of days amid an operational meltdown.

Ina statement to Newsweek, Southwest Airlines told passengers that it “anticipates some schedule adjustments” as the system malfunction issue is resolved.

United Airlines told Newsweek that it has resumed operations following the ground stop. Customers, however, may still see some delays and cancellations as the airline works to restore its schedule, United said.

United also activated a travel waiver for “any customers who need to change their plans, including offering refunds for customers who no longer want to travel.”

A ground stop for all inbound and outbound flights at Chicago O’Hare International and Chicago Midway International Airports continue following the FAA system failure.

Leadership at both airports say they remain in touch with the FAA and their airline partners to “minimize impacts to flight operations.”

Normal air operations are resuming “gradually” across the country, the FAA said in an update at 8:50 a.m. ET.

The cause of the problem is still under investigation.

“Flights to US airports have been groundstopped by the FAA until at least 09:30 ET (1430 UTC),” FlightRadar wrote.

“This means any flight not in the air already will not be allowed to depart. Image: flights over US now compared to same time last week. About 700 fewer flights airborne now.”

By 8:30 a.m. ET, more than 3,700 flights into and out of the U.S. were delayed, according to flight tracking website FlightAware. The delays were spread across several airlines, including United, American and Southwest.

The current delays and cancellations come at a difficult time for air travel in the U.S. as hundreds of flights were canceled and thousands delayed over the Christmas holiday period due to bad weather.

The FAA wrote that departures are resuming at Newark Liberty International Airport in New Jersey and Hartsfield-Jackson Atlanta International Airport in Georgia “due to air traffic congestion in those areas.”

The agency expects departures to resume at other airports at 9 a.m. ET as previously announced.

Writing on Twitter, Buttigieg said: “I have been in touch with FAA this morning about an outage affecting a key system for providing safety information to pilots.

“FAA is working to resolve this issue swiftly and safely so that air traffic can resume normal operations, and will continue to provide updates.”

“The President has been briefed by the Secretary of Transportation this morning on the FAA system outage,” wrote White House Press Secretary Karine Jean-Pierre on Twitter.

“There is no evidence of a cyberattack at this point, but the President directed DOT to conduct a full investigation into the causes. The FAA will provide regular updates.”

The order to temporarily suspend flights across the country came directly from the FAA, which asked in a statement for time “to allow the agency to validate the integrity of flight and safety information.” Several airlines are currently affected by the FAA system outage.

In its latest update on the unfolding situation, the FAA wrote that the system is “beginning to come back on line.” The agency is “still working to fully restore the Notice to Air Missions system.”

United Airlines told Newsweek that “United has temporarily delayed all domestic flights and will issue an update when we learn more from the FAA.”

It confirmed that “the FAA system that sends out important real-time flight hazards & restrictions to all commercial airline pilots - Notice to Air Missions (NOTAM) - is currently suffering a nationwide outage.”

American Airlines told Newsweek: “The Federal Aviation Administration is experiencing an outage with its NOTAM (Notice to Air Missions) system, which provides critical flight safety operational information. We are closely monitoring the situation, which impacts all airlines, and working with the FAA to minimize disruption to our operation and customers. We encourage customers to check aa.com for the latest flight information.”

Newsweek has also contacted Southwest Airlines for comment.