A highly automated military plane that crashed in Spain appears to have been hit by a software fault.

The Airbus A400M, which was being tested before going into active service, crashed shortly after take off last month killing four out of the six crew members. The crash happened when three of the four turboprop engines froze at the same time.

The automation feature in question is not so much about controlling the plane, but rather about taking the pilot’s controls and turning that into action in the most effective and accurate manner. Investigators believe the problem lay with the way the plane interprets the current engine torque levels to spin the propellers precisely in line with the pilot commands.

Reuters reports that investigators believe the relevant data files, wich detail the relationship between torque and propeller spin for the particular engines, were missing. They believe they may somehow have been deleted during software installation. Without the relevant data, the system simply stopped spinning the propellers.

It seems this in turn triggered two further weaknesses, which combined to cause the deadly results. The first was that although the plane system is designed to adjust if an engine fails, this safeguard wasn’t set up to deal with the possibility of three engines “failing” simultaneously for the same reason.

The second flaw was that an alert system designed to warn the pilot that the automated propeller control had failed was not set up to give the alert until the plane was at least 400 feet in the air. In this case it appears likely that the failure was already an issue at takeoff.

An Airbus spokesperson refused to address specific points raised by Reuters but instead said ” Safety is our first priority and we will do all that is necessary to get the full picture of what could have led to this tragic accident and take the necessary action.”