Air traffic controllers experienced a temporary communication breakdown with planes arriving and departing from Newark Liberty International Airport last week, leading to significant delays at the busy travel hub.

Last Monday, flights were halted at Newark Airport due to an equipment failure within the Federal Aviation Administration. The agency has yet to provide details on the nature of the malfunction.

During the event, air traffic controllers from Area C of the Philadelphia TRACON, which manages flight paths in and out of Newark, lost the ability to see, hear, or communicate with the aircraft, as stated by the National Air Traffic Controllers Association (NATCA). These controllers are responsible for directing the flow of planes to and from Newark Airport.

It’s not clear at this time how long the air traffic controllers lost communication with the aircraft.

The event led the air traffic controllers to take leave under the Federal Employees Compensation Act, which covers federal employees who are physically injured or experience a traumatic event on the job, according to NATCA, a union that represents around 20,000 aviation professionals.

While the union did not elaborate on what caused the communication loss, it said it supports Department of Transportation Secretary Sean Duffy’s “efforts to upgrade and modernize our nation’s air traffic control systems and infrastructure.”

Since the incident, delays and cancelations of flights traveling into and out of Newark airport have been constant. Airlines, including United, have had to cancel hundreds of flights, and dozens of others were diverted to other airports.

Many issues contributed to the disruptions, including understaffing, several instances of technical equipment failure and construction. On Monday afternoon, over 300 flights traveling to and from Newark were delayed, and more than 150 were canceled, according to FlightAware.com.

Bill Walker was one of the passengers to have his flight delayed on Monday.

“We were delayed almost 3 hours in Seattle because they couldn’t get air clearance to come into Newark,” Walker told NBC News.

Other travelers weren’t as lucky.

Ireland Hernandez, who was trying to get back home to Mexico, had an afternoon flight to Houston that got canceled.

“We came here just for the weekend and vacation, and we were planning to go back to Los Angeles,” Hernandez said. “But we have this connection flight from Newark to Dallas, and then we were driving to Mexico.”

She was traveling with her friend, Sarah Arce. They said they struggled to get answers or support from airline staff.

“They said ‘we can’t give you a voucher,’” Arce said.

United CEO Scott Kirby said that the Philadelphia TRACON “has been chronically understaffed for years,” adding that “Newark airport cannot handle the number of planes that are scheduled to operate there in the weeks and months ahead.”

A shortage in air traffic controllers is an ongoing, nationwide problem. According to Duffy, there are some 3,000 vacant positions nationwide and he’s planning to roll out a strategy to incentivize the labor force.

The Federal Aviation ministration did not immediately respond to a request for comment.