[Source: Reuters]

U.S. authorities said on Thursday it was not yet clear why a regional jet and a U.S. Army helicopter collided at a Washington airport, killing 67 people in the deadliest U.S. air disaster in more than 20 years.

Investigators recovered the so-called black boxes from the plane, an American Airlines Bombardier jet carrying 60 passengers and four crew members, which collided with an Army Black Hawk helicopter and crashed into the Potomac River as it prepared to land at Ronald Reagan Washington National Airport on Wednesday night.

No one survived. Names of all the victims have not been released, but they included promising young figure skaters and people from Kansas, where the flight originated.

Senator Maria Cantwell said that the dead also included citizens from Russia, the Philippines and Germany, and Chinese state media Xinhua reported two Chinese citizens had been killed in the crash.

Investigators recovered the cockpit voice recorder and flight data recorder from the Bombardier CRJ700 passenger jet, the National Transportation Safety Board said. A preliminary report about the incident is expected within 30 days.

NTSB member Todd Inman said the military helicopter was equipped “with some form of recording devices and those will be read either by DOD or by us.” He said NTSB and DOD have agreements to share data on those recorders.

The agency has begun collecting wreckage, including portions of the helicopter, and is storing it at a hangar at Reagan National.

Washington’s fire and emergency department said its divers had searched all accessible areas and would conduct additional searches to locate aircraft components on Friday.

U.S. Transportation Secretary Sean Duffy said both aircraft had been flying standard flight patterns on Wednesday and there had been no breakdown in communication.

“Everything was routine up to the point of the accident,” Virginia Governor Glenn Youngkin told Reuters. More than half of the bodies had been recovered, he said. The airport is just across the river from Washington in Virginia.

President Donald Trump, without evidence, said that federal diversity efforts could have been a factor, reiterating a theme that has become a focus of his presidency. Rights groups and Democrats said he was politicizing the disaster.

Trump also criticized the helicopter pilots and suggested air traffic controllers were to blame.

Radio communications showed that air traffic controllers alerted the helicopter about the approaching jet and ordered it to change course.

One controller rather than two was handling local plane and helicopter traffic on Wednesday night at Reagan National, a situation deemed “not normal” but considered adequate for lower volumes of traffic, according to a person briefed on the matter.

The decision to combine duties in the evening is not uncommon, the source said. The New York Times first reported the “not normal” designation.

A shortage of air traffic controllers in the United States in recent years has spurred safety concerns.

At several facilities, controllers work mandatory overtime and six-day work weeks to cover shortages. The Federal Aviation Administration has about 3,000 fewer controllers than it says it needs.