The Federal Aviation Administration says staffing levels were below the "normal" amount when the deadly plane crash occurred over Washington, D.C. ... and, it looks like it was a decision made by a supervisor.

The New York Times -- citing an anonymous source -- says an air traffic control supervisor combined the responsibilities of handling both helicopters and plane flight paths before the usual time ... though it's currently not clear why.

Basically, before 9:30 PM, Reagan Washington National Airport has two controllers -- one to handle the choppers and the other to handle the airplanes -- and, after 9:30 they transition to only one person because traffic lessens significantly.

However, NYT says the supervisor combined the roles before 9:30 ... a deviation from the usual routine. We've reached out to Reagan National to confirm, but either way, the FAA has already said staffers were "not normal for the time of day and volume of traffic."

There's been tons of speculation around what caused the crash ... further spurred by President Donald Trump, who suggested diversity-hiring initiatives led to the tragic incident.

As you know ... an American Airlines flight carrying 60 passengers and four crew members crashed into a Black Hawk helicopter with three military personnel on board. Officials expect there are no survivors.

Harrowing audio of an air traffic controller urgently calling out to the helicopter moments before the crash -- telling the crew to maneuver behind the plane -- has also emerged.

The FAA's investigation remains ongoing ... they've just recovered two black boxes from the plane -- so, we may soon have a better explanation about what went wrong.