Hundreds of Alaska Airlines and United Airlines flights remained canceled Monday in the wake of the Federal Aviation Administration’s decision to ground all Boeing 737 Max 9 aircraft.

According to data from tracking site FlightAware, 220 United flights are affected. About 140 Alaska Airlines flights — 20% of the Seattle-based carrier’s routes — have been affected, as of 9:55 a.m. ET Monday morning.

The cancellations are concentrated at Alaska’s Seattle-Tacoma International Airport hub; other airports seeing outsized cancelations include Newark-Liberty International Airport, Denver International Airport, and Chicago-O’Hare International Airport.

In a statement on its website late Sunday, Alaska said the FAA’s grounding order had “significantly impacted our operation” and that cancellations would continue through the first half of the week. Already, the 170 flights it canceled Sunday had affected roughly 25,000 travelers, it said.

Both Alaska and United said affected passengers would be rerouted to their destinations.

The decision to ground the Boeing aircraft came after a panel blew out of an Alaska plane’s fuselage with 171 passengers and six crew members onboard Friday, forcing the aircraft to make an emergency landing.

Late Sunday, the National Transportation Safety Board reported it had recovered the panel in question. It continues to probe whether other Max 9’s could be prone to the same issue.