A single malfunctioning Federal Aviation Administration router at a radio site in rural Arkansas appears to have triggered the nationwide air travel chaos that grounded flights across the United States on Friday, according to a new report.

The trouble originated at an FAA facility in El Dorado, Arkansas, where a newly installed router overwhelmed the network with excessive data traffic, The Wall Street Journal reported. The surge disrupted vital communications between pilots and air traffic controllers.

More than 3,500 flights were grounded during the shutdown, which lasted roughly five hours. Tens of thousands of travelers were stranded as delays and cancellations spread across the country.

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The router reportedly began bombarding other devices throughout the air traffic control network with connection attempts. Those repeated attempts interfered with communications involving aircraft and disrupted radar feeds used by the system.

Rebecca Guy, the FAA’s technology chief, described the unusually heavy network activity on Saturday. “We saw two places that were actually pinging our network at a very high rate, a thousand connections over every four to five seconds,” Guy said.

Investigators are examining whether defective equipment or installation mistakes by contractor L3Harris caused the failure. L3Harris operates and owns the network involved in the incident, according to Secretary of Transportation Sean Duffy.

Duffy confirmed the routing problem during a Sunday appearance on Face the Nation. He said the FAA was at the site and that the FBI had also offered support during the investigation.

“We actually have both routers with L3Harris, who operates and owns our network. FAA is on site as well. FBI is offering support. We’re looking at that right now, and when we know what the root cause of this issue was, was it the install from L3Harris? Was it the router?” Duffy said.

Duffy promised that the findings would be disclosed once officials determine what happened. “I’m going to tell you and the American people what we find.”

The failure occurred during routine maintenance intended to replace aging copper telecommunications equipment with fiber optic cables, Duffy said. The work is part of a broader effort to modernize the FAA’s outdated infrastructure.

The Arkansas router is not the only equipment under examination. A radar station in Amarillo, located in the Texas panhandle, is also being considered as a possible contributor to the outage.

El Dorado, the location of the FAA radio site where the problem reportedly began, is a city of about 22,000 people. It sits approximately 210 miles northeast of Dallas, making it an unlikely center for a disruption that rippled through air travel nationwide.

L3Harris said it is assisting the FAA’s investigation into the outage. The company also said it has completed more than 70% of the planned conversion from legacy copper wiring to fiber optic technology.

The contractor has worked with the FAA on network modernization efforts for more than two decades. Officials are now trying to determine whether the newly installed router itself failed or whether the trouble stemmed from how the equipment was installed.

FULL INTERVIEW: Transportation Secretary Sean Duffy tells @margbrennan that air travel should return to full capacity by the end of the day or Monday, following Friday's nationwide ground stop. https://t.co/AqvAT0I0IH

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The FAA said there was no evidence that a cyberattack caused the widespread disruption. For now, the investigation remains centered on the equipment, the installation process and the network activity tied to the Arkansas site, with the Amarillo radar station also under review.