The Army pilots whose Black Hawk helicopter crashed into a passenger jet over the Potomac River on Jan. 29 may have been misled by their instruments, causing them to believe they were at a safer, lower altitude when they were actually headed straight into the jet’s path, according to evidence that federal investigators unveiled on Wednesday.

That revelation came as the National Transportation Safety Board began three days of public hearings into the midair collision that killed 67 people, the first fatal crash involving a major American airline in 15 years.

Those hearings revealed a series of malfunctions and fateful decisions that night that heightened the dangers of an already crowded airspace over the river near Washington’s Ronald Reagan National Airport, crisscrossed by passenger jets and helicopters flown by the military and local police.

Inside the helicopter, for instance, investigators said that the Black Hawk’s instruments might have shown the pilots were flying 80 to 100 feet below their actual position. The area in which the helicopter was flying did not have much room for error. Helicopters there were expected to fly in a narrow band between 100 and 200 feet above the Potomac.

“Altitude callouts showed a possible lower than actual altitude understanding by the crew,” Marie Moler, a mechanical engineer who provided technical expertise for the agency, said during the hearing.

Inside the airport’s control tower that night, investigators found, a single controller was handling both helicopter and airplane traffic at the time of the 8:48 p.m. crash, though the jobs are not typically combined at that hour.