Ronald Reagan Washington National Airport (DCA) remained shut down on Thursday morning after an American Airlines regional jet fatally collided mid-air with a military helicopter just outside the airport on Wednesday night. As of early Thursday morning, more than 500 flights had been canceled in and out of DCA, according to flight tracker FlightAware.

All flights were halted at the Washington, D.C.-area airport and aren’t expected to resume until at least 11 a.m. on Thursday, according to the airport. The airport’s terminals remain open.

“All airport roads and terminals are open,” the airport wrote in a post on X, formerly known as Twitter. “Some flights have been delayed or cancelled, so passengers are encouraged to check with their airline for specific flight information.”

The incident occurred at about 9 p.m. ET on Wednesday when American Eagle Flight 5342 collided midair while attempting to land at the airport with a Sikorsky H-60 helicopter, according to the Federal Aviation Authority (FAA). The flight was operated on a Bombardier CRJ700 aircraft by regional carrier PSA Airlines, wholly owned by American, and was headed from Wichita, Kansas, to Washington, D.C.

There were 60 passengers and four crew members on board the American Airlines aircraft, according to a statement from the carrier. The Associated Press reports that a military helicopter, which also had several passengers on board, was on a training flight at the time.

As of Thursday morning, PSA Airlines was forced to cancel more than 280 flights and American Airlines was forced to cancel an additional 165 flights, according to FlightAware.

“First, and most importantly, I’d like to express our deep sorrow about these events,” Robert Isom, CEO of American Airlines, said in a video address posted to the airline’s website. “This is a difficult day for all of us at American Airlines and our efforts now are focused entirely on the needs of our passengers, crew members, partners, first responders, along with their families and loved ones.”

The National Transportation Safety Board (NTSB) and the Federal Aviation Administration (FAA) have launched an investigation into the crash, according to the FAA. Isom said the airline was “cooperating fully” with the investigation and “we want to learn everything we can about today’s events.”

The bodies of at least 28 people have been recovered from the frigid Potomac River, and while officials are still searching for casualties, they don’t expect to find any survivors, the AP reported. If true, that would make this the deadliest U.S. air crash in nearly 24 years.