The crew of the Black Hawk helicopter involved in a midair collision with an American Airlines jet above the Potomac River last month may not have heard a key directive from air traffic control and might have been viewing faulty flight data, according to investigators.

Jennifer Homendy, chair of the National Transportation Safety Board, said air traffic control sent a message to the Army pilots of the Black Hawk helicopter to “pass behind the” jet.

But, because of the timing, the crew of the helicopter may have missed that key phrase, as they were keying the microphone to communicate back to air traffic controllers.

Audio from the cockpit voice recorder indicates the instructor pilot was monitoring and communicating on the radio, Homendy said, and another pilot was flying the aircraft.

Just before 8:44 p.m., the instructor pilot indicated the helicopter was at 400 feet and the pilot flying the helicopter indicated they were at 300 feet, Homendy said. There was no discussion about the discrepancy.

Investigators are also looking at the possibility there was “bad data” and that the pilots in the cockpit of the helicopter may have been seeing an altitude that differed from radio altitude data captured by the flight data recorder.

Homendy said investigators are confident the radio altitude of the Black Hawk helicopter was 278 feet at the time of the crash.

“I want to caution this does not mean this is what the Black Hawk crew was seeing on the barometric altimeters in the cockpit,” she said.

Homendy said investigators still “have a lot of work to do” before the investigation into the deadly crash is complete.

The Jan. 29 crash above the Potomac River left no survivors. There were 64 people aboard the American Airlines flight and three on the Black Hawk helicopter on a training mission.

The dead include several Marylanders, and the crash sent shock waves through the state’s tight-knit figure skating community — many on the plane were returning from a development camp in Kansas.

Given the nature of the helicopter flight — which was a combined annual check and night vision goggle check flight — Homendy said the Army pilots were likely wearing their night vision goggles at the time of the crash.

If the crew were going to remove their night vision goggles, Homendy said, there would have been a discussion it. There was no evidence from the cockpit voice recorder that there was such a discussion of the goggles being removed.

The investigation team will be conducting a visibility study for both aircraft to determine what the crews in both cockpits may have been able to see at the time of the collision.

One air traffic controller was working two positions at the time of the crash, according to a Federal Aviation Administration report that was obtained by the Associated Press.

Shortly after the crash, President Donald Trump asserted the FAA’s efforts to recruit a more diverse staff had made air travel less safe. Trump did not provide evidence to support his claim.

One of the three soldiers on the helicopter training mission was Chief Warrant Officer 2 Andrew Loyd Eaves, of Great Mills, Maryland. Eaves was a Navy veteran turned Army pilot. He served in the Navy from 2007-2017 and transitioned to his role as an Army pilot in September 2017.

Several months before the crash, Maryland’s Sens. Chris Van Hollen and Ben Cardin (who has since retired) voted against a bill reauthorizing the FAA because it added 10 additional flight routes at Reagan Airport.

Ronald Reagan Washington National Airport is used by members of Congress flying to and from D.C. The two Maryland senators said the airport was overburdened.

“The Senate has prioritized the convenience of a few senators over the safe operations of the busiest runway in the United States,” Cardin said in a statement explaining his vote at the time. “I could not support this.”

Homendy reiterated that air travel is “incredibly safe” in the United States.

“Aviation is the safest mode of transportation. I say this often. You are at greater risk getting into your car to and from the airport,” she said.