The black boxes have been pulled from the wreckage of the American Airlines passenger jet that collided with an Army helicopter, killing 67, amid concerning new reports about possible causes of the crash. 

The Blackhawk helicopter had reportedly flown higher than its approved path and was a half mile off course, and the airline pilots were likely unable to see the craft as they were making a turn to a different approach requested by the air traffic controller for its landing at Reagan National Airport, according to flight recording and a preliminary internal report from the Federal Aviation Administration.

The helicopter also was above 300 feet when it should have been below 200 feet. 

According to people briefed on the incident who spoke with the New York Times, the helicopter pilot confirmed with air traffic control that they had visual confirmation of the plane from the controller, who told them to follow the route to go behind the plane but did not follow instructions.

Standard protocol for helicopters flying through busy commercial airspace requires pilots to gain approval from air traffic control. The pilots involved in Wednesday’s crash had permission to use a predetermined route that prevents copters from flying higher than 200 feet and along a bank on the east side of the Potomac River, which would have led to them avoiding the jet.

The two flight recorders recovered from the wreckage of the American Airlines plane will be sent to National Transportation Safety Board investigators to help them piece together how the accident had unfolded.

“We look at the human, the machine, and the environment,” NTSB Chairwoman Jennifer Homendy told reporters on Thursday. “We will look at all the humans that were involved in this accident. We will look at the aircraft. We will look at the helicopter. We will look at the environment in which they were operating in. That is standard.”

The official also told the newspaper that the Blackhawk’s pilots had flown the route before and were aware of the usual protocols.

Typically, air traffic control at Reagan National Airport has one controller to handle the plane and a second to handle helicopter traffic until 9:30 p.m., but on the night of the crash, one of the controllers was allowed to leave early. The crash occurred around 9 p.m.

America also has an ongoing controller shortage, with only 10,800 certified operators, short of the required 14,000, according to CBS News.

“It is not uncommon to us to routinely combine positions, de-combine positions,” National Air Traffic Controllers Association President Nick Daniels said on “CBS Mornings.” 

“There is usually someone in a supervisory position looking at the overall workload and complexity in order to make those determinations of what positions we need open and when we need them open,” he added. “The number of air traffic controllers that are needed is always a determining factor, and quite often, if we don’t have enough people, we begin reducing the number of aircraft we can officially move and safely move through the airspace.”