NEWARK, N.J. (AP) — Flight delays and cancellations persisted at Newark Liberty International Airport Monday, with Senate Minority Leader Chuck Schumer calling for an investigation into the “chaos” sparked by an air traffic controller shortage and thick cloud cover.

Schumer, a Democrat from New York, said he wants an inspector general’s investigation into the delays and cancellations to prevent them from getting worse or spreading. New Jersey Democratic Gov. Phil Murphy called the delays “completely and utterly unacceptable” in a post on X, and said Transportation Secretary Sean Duffy is “committed” to hiring more air traffic controllers.

The delays at the busy airport outside New York City made headlines over the weekend as United Airlines said it was cutting 35 daily flights from its schedule.

As of Monday, the Federal Aviation Administration attributed arriving flight delays of nearly four hours to a combination of staffing and cloud cover.

Here’s a closer look at what to know about the delays.

What’s behind the delays at Newark?

The airport faced disruptions last week as well.

United CEO Scott Kirby said in a letter to customers over the weekend that the technology used to manage planes at the New Jersey airport failed more than once in recent days.

The flight delays, cancellations and diversions that the equipment problems caused were compounded when more than one-fifth of Newark’s traffic controllers “walked off the job,” he said.

Faulting the Federal Aviation Administration’s alleged failure to address “long-simmering” challenges related to the air-traffic control system, United Airlines cut 35 daily flights from its Newark schedule starting Saturday.

Messages seeking comment were left with the air traffic controllers labor union and the federal Transportation Department, which oversees the FAA.

What’s the broader issue with air traffic control?

The Trump administration has said an “obsolete” air traffic control system needs to be fixed.

The administration says it’s been trying to “supercharge” the air traffic controller workforce and address the nation’s shortage of controllers. U.S. Transportation Secretary Sean Duffy last week announced a program to recruit new controllers and give existing ones incentives not to retire.

The National Air Traffic Controllers Association, a workers’ union, said at the time that those moves could help address staffing shortages, but it also said the system is “ long overdue for technology and infrastructure upgrades.”

Uncertainty at play?

United’s decision to pare back its flight schedule in Newark comes at an already uncertain period for U.S. airlines. Potential customers across the industry are reconsidering whether to fly for work or for vacation given all the unknowns about what President Donald Trump’s trade war will do to the economy.

Uncertainty is so high that United recently made the unusual move of offering two separate forecasts for how it could perform financially this year: one if there were a recession and one if not.

United flies to 76 U.S. cities and 81 international destinations from Newark.