The FAA’s Air traffic Control System Command Center reported that planes were grounded at all airports, but according to United Airlines, the issue did not "directly affect" any flights already in the air.
"The overwhelming majority of flights that were affected were flights that had not departed airports,” United spokesman Charles Hobart told ABC News.
“We experienced a network connectivity issue this morning. We are working to resolve this and apologize to our customers for any inconvenience,” United said in a statement.
The airline also later released an online video apology to passengers.
United is one of the world's largest airlines with nine hubs in the United States. The company says it is providing a waiver for customers who were unable to change their flight plans.
The airline and law enforcement official both confirmed to ABC News that the network connectivity issue was not due to hacking or a cyber-attack.
United officials say an issue with a router degraded network connectivity, impacting the company's ability to communicate data to aircraft on the ground.
Some passengers, however, complained of delays: