Federal investigators in the United States are trying to determine the cause of a deadly plane crash over Washington DC.

Officials have reportedly recovered the plane's black boxes, which may offer some clues.

The black boxes are to be analyzed at the National Transportation Safety Board lab.

The incident occurred Wednesday when a passenger jet collided with a military helicopter near Ronald Reagan National Airport.

All 67 people aboard both aircraft are feared dead.
The plane and the helicopter came down in the nearby Potomac River.

US President Donald Trump said he has appointed an acting commissioner of the Federal Aviation Administration to lead the investigation.

The New York Times has reported that an internal report from the US Federal Aviation Administration suggests staffing at the air traffic control tower at the time of the collision was "not normal for the time of the day and volume of traffic."

It said that according to the report, the air traffic controller who was handling helicopters in the airport's vicinity was also instructing planes that were landing and departing from its runways -- jobs typically assigned to two people.

The New York Times added that many air control towers in the country have been understaffed for years.

Trump's nominee for secretary of the Army, Daniel Driscoll, commented on the risks involving training missions in a Senate committee hearing on Thursday.

He said, "I think we might need to look at where is an appropriate time to take training risk, and it may not be near an airport like Reagan."