On January 29, a routine flight above Washington DC’s skies turned catastrophic when a Black Hawk helicopter collided with an American Airlines plane. Both aircraft plunged into the Potomac River after the collision, claiming 67 lives in the deadliest U.S. aviation disaster since 2001. Recovery efforts continued until February 4, when officials confirmed all victims’ bodies had been retrieved from the water.

Now, weeks after the tragedy, black box data about the helicopter is revealing the sequence of events that led to the fatal crash. One Black Hawk pilot is heard saying the helicopter was at 300 feet, while the other said 400 feet. According to National Transportation Safety Board (NTSB) Chairwoman Jennifer Homendy, it’s possible the pilots had “bad data.”

“We do believe that there’s inconsistency with data from the altimeters in the cockpit. So it’s possible that what they were seeing is not matching up with what’s recorded on the flight data recorder,” Homendy tells NBC News. “At this time we don’t know why there was a discrepancy between the two.”

The pilots may also not have heard instructions to pass behind the civilian plane. The words “pass behind the” on the recording are unclear and were likely obscured as the crew was pressing the record button to send their own message.

“We’re only a couple weeks out from the crash. We have a lot of work to do,”  Homendy said. “The NTSB expects it will take more than a year to produce a comprehensive report explaining how these factors combined to create this devastating accident.”