U.S. flights were slowly resuming departures and a ground stop was lifted after the Federal Aviation Administration (FAA) scrambled to fix a system outage overnight that had forced a halt to all U.S. departing flights.

More than 6,000 flights were delayed and nearly 1,000 canceled according to the FlightAware website as officials said it will take hours to recover from the halt. The numbers were still rising.

The cause of the problem with a pilot-alerting system, which delayed thousands of flights in the United States, was unclear, but U.S. officials said they had so far found no evidence of a cyberattack.

The outage occurred at a typically slow time after the holiday travel season, but demand remains strong as travel continues to recover to near pre-pandemic levels.

"Normal air traffic operations are resuming gradually across the U.S. following an overnight outage to the Notice to Air Missions system that provides safety info to flight crews. The ground stop has been lifted. We continue to look into the cause of the initial problem," the FAA Tweeted.

RIPPLE EFFECTS

Captain Chris Torres, vice president of the Allied Pilots Association, said the outage could impact traffic through Friday.

"This thing was lifted at 9 a.m. Eastern. That doesn’t mean the problem stops at 9 a.m. This is going to cause ripple effects," said Torres, whose members fly for American Airlines.

"The end result of this is going to be very similar to big weather events."

The FAA had earlier ordered airlines to pause all domestic departures after its pilot alerting system crashed and the agency had to perform a hard reset around 2 a.m., officials said.

Flights already in the air were allowed to continue to their destinations.

U.S. President Joe Biden ordered the Transportation Department to investigate and said the cause of the failure was unknown. Asked if a cyber attack was behind the outage, Biden told reporters, "We don't know."

Transportation Secretary Pete Buttigieg pledged a "process to determine root causes and recommend next steps."

Shares of U.S. carriers initially fell in Wednesday's premarket trading, but most rallied after the market opened to positive territory as flights resumed.

After dropping more the 19% last year - its third consecutive year of decline - the S&P 500 airlines index has gotten off to a strong start this year, up 15.5%, as travelers return to the skies.

Southwest Airlines shares were flat, while Delta Air Lines Inc, United Airlines and American Airlines shares rose. JetBlue and Spirit each rose about 2%.

A trade group representing the U.S. travel industry, including airlines, called the FAA system failure "catastrophic."

"America’s transportation network desperately needs significant upgrades," Geoff Freeman, president of the U.S. Travel Association, said in a statement.

"We call on federal policymakers to modernize our vital air travel infrastructure.”

The number of flights impacted rose even after the ground stop was canceled. One issue airlines are facing is trying to get planes in and out of crowded gates, which is causing further delays.

At an airport in Greenville, South Carolina, Justin Kennedy abandoned a work trip to nearby Charlotte. He said confusion reigned as airline employees weren't aware what the FAA was saying, and many passengers were initially unaware of the delays.

"I sat in a Chick-fil-A dining area that had a good view of the TSA exit," the 30-year-old information technology employee said.

"I saw at least four people sprinting to gates because they thought they were going to miss their flight, only to come back to the food court, out of breath."

COMPLETELY UNACCEPTABLE'

For U.S. travelers flying great distances, the alternatives are few. Driving distances are too great, and the country's passenger rail network is thin compared to those in Europe and Asia.

Senate Commerce Committee chair Maria Cantwell, a Democrat, said the panel would investigate.

“We will be looking into what caused this outage and how redundancy plays a role in preventing future outages," she said.

Republican Senator Ted Cruz called the failure "completely unacceptable" and said the issue should lead to reforms as part of the FAA reauthorization due by September.

FAA's system outage comes weeks after an operational meltdown at Southwest at the end of last year left thousands stranded.

A severe winter storm right before Christmas, coupled with the Texas-based carrier's dated technology, led to over 16,000 flight cancellations.

The DOT, FAA's parent agency, criticized Southwest's failures and pressured the airline to compensate passengers.

There is no legal requirement that the FAA must compensate passengers for flight delays caused by agency computer issues.

The FAA suffered another significant computer issue on Jan. 2 that led to significant delays in Florida flights.

A problem with the functioning of a system known as the En Route Automation Modernization (ERAM) used to control air traffic prompted the FAA to issue a ground stop order, slowing traffic into Florida airports.

A NOTAM is a notice containing information essential to personnel concerned with flight operations but not known far enough in advance to be publicized by other means.

A ground stop is an air traffic control measure that slows or halts aircraft at a given airport.

A total of 21,464 U.S. flights were scheduled to depart Wednesday with a capacity of nearly 2.9 million passengers, data from Cirium showed.

Rodney Allen was on his way with friends to vacation in Puerto Rico from Cincinnati but got stranded in Newark.

"Once we landed, passengers on the plane were saying flights were grounded," the 25-year-old entrepreneur said.

He still had the option to check in to his flight to Puerto Rico, but his friends had been offered travel credits.