U.S. Air Force investigators have determined that an MQ-1B Predator crashed last year after its controllers lost a datalink with the unmanned aircraft, the service’s Air Combat Command announced June 5.

According to the command, the datalink was lost while the Predator was performing a combat support mission on March 8, 2016, in U.S. Central Command’s area of responsibility. While control of the aircraft was ultimately reestablished, the ground control station did not alert the aircrew. As a result, the crew did not adjust its flight controls to sustain flight, leading to “to an inadvertent descent and eventual impact with the ground.”

Investigators concluded that unclear guidance on emergency procedures “substantially” contributed to the crash, the command said. They also found that the unplanned presence of an instructor pilot created confusion about who was in charge during the recovery effort.

The aircraft, built by General Atomics Aeronautical Systems Inc. (GA-ASI), and its munitions were estimated to cost $4.2 million.