The black boxes for a commercial flight that crashed into a military helicopter in Washington DC have been located, as questions mount about staffing and other close calls at the airport where the plane was landing.

Normally two people manage air traffic control for helicopters and airplanes flying in the area - one of the most controlled airspaces in the world - but only one person was doing so on Wednesday at the time of the crash, according to sources cited by the BBC's US partner CBS News.

Officials are still investigating the cause of the incident that killed all 67 people aboard the two aircraft.

The National Transportation Safety Board (NTSB) said a preliminary report will be issued in 30 days.

The flight data recorder and the cockpit voice recorder, known as the black boxes, can help offer clues to investigators about what may have gone wrong on the flight.

The boxes will be transported to the NTSB lab, nearby the site of the crash, and analysed, according to CBS.

The air traffic control staffing numbers, first reported by the New York Times, was noted as "not normal", according to a preliminary Federal Aviation Administration (FAA) report.

The federal government has struggled for years to fill certain key positions at the FAA.

The type of staffing that had one air traffic control worker managing both the helicopters and planes at the Regan Washington National Airport on Wednesday night is reportedly not uncommon and did not breach guidelines.

Divers spent most of Thursday swimming through the icy waters of the Potomac River, searching for the bodies of victims.

The search was suspended on Thursday evening due to dangerous conditions.

There were 64 passengers aboard the American Airlines flight that collided with an Army Black Hawk helicopter that was on a training mission flight. Three soldiers were on board.

Teams have so far recovered 27 bodies from the plane and one from the helicopter.

At a White House briefing on Thursday, President Donald Trump began with a moment of silence and a prayer for victims.

He said "we can only begin to imagine the agony that you're all feeling", adding, "our hearts are shattered alongside yours".

Trump speculated on the cause of the collision, suggesting without evidence that lower hiring standards for air traffic controllers in the FAA under previous administrations run by Democrats may have been a factor.

Later on Thursday, he signed a memorandum to end diversity, equity and inclusion (DEI) practices in the aviation sector.

DEI programmes aim to promote participation in workplaces by people from a range of backgrounds. Backers say they address historical or ongoing discrimination and underrepresentation but critics argue they can themselves be discriminatory.

Trump also signed an executive order to appoint a new head of the FAA.

The fatal incident happened at about 21:00 local time (02:00 GMT) on Wednesday, when a PSA Airlines jet operating as American Airlines 5342 collided mid-air with a US Army helicopter as it approached Ronald Reagan Washington National Airport.

Both aircraft careened into the Potomac River. The passenger plane broke into multiple pieces and sank several feet into the water, while the helicopter ended up upside down in the river.

The plane, a Bombardier CRJ700, had departed from Wichita, Kansas, and was carrying dozens of passengers, including a pair of young figure skaters, their mothers, and two Russian coaches.

Kremlin spokesman Dmitry Peskov was quoted in Russia media saying: "There were other of our fellow citizens on board. It's bad news today from Washington.

"We are sorry and send our condolences to families and friends."

The helicopter was a Sikorsky H-60 that took off from Fort Belvoir in Virginia with three soldiers on board, and belonged to B Company, 12th Aviation Battalion.

"It's a tragedy, a horrible loss of life for those 64 souls on that civilian airliner, and of course the three soldiers in that Black Hawk [helicopter]," said new defence secretary Pete Hegseth on Wednesday. (BBC)

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