President Joe Biden has ordered an investigation into an FAA system outage that grounded flights across the country Wednesday morning, citing an unknown cause.

Biden told reporters at the White House that he had spoken with Transportation Secretary Pete Buttigieg and that they should have a good idea of what caused the outage in a couple of hours.

“We’ll respond at that time,” Biden said. Asked if the outage was caused by a cyberattack, he said, “We don’t know.”

“They don’t know what the cause is,” Biden said. “Aircraft can still land safely just not take off right now. We don’t know what the cause of it is.”

White House press secretary Karine Jean-Pierre said earlier in a Twitter post that there was no evidence of a cyber attack at this time.

U.S. flights were grounded or delayed on Wednesday as the FAA scrambled to fix a system outage, with passengers told to check with airlines for updates.

The FAA said it had ordered airlines to pause all domestic departures until 9 a.m. ET (1400 GMT) to allow the agency to validate the integrity of flight and safety information.