The American Airlines plane and a Black Hawk helicopter collided near Ronald Reagan National Airport in January, killing all 67 people onboard. Both aircraft crashed into the Potomac River, triggering an investigation.

Investigators found the helicopter crew may have had “bad data” about their altitude. One pilot believed they were at 400 feet, another at 300 feet. NTSB Chair Jennifer Homendy said, “We are looking at the possibility of there may be bad data.”

Air traffic control instructed the helicopter to stay behind the plane, but the crew may not have heard the transmission. Homendy noted the pilot “may have keyed her radio at the same second and stepped on the transmission from ATC.”

The Black Hawk’s ADS-B system, which improves tracking, was switched off during the training mission. Senator Ted Cruz said, “Unless there was a compelling national security reason for turning it off, that does not seem justified.”

Brig. Gen. Matthew Braman summed it up: “There were multiple things that, had any one of them changed, it could have well changed the outcome of that evening.” Errors in communication and tracking contributed to the deadly crash.