(Reuters) – The crew of an A400M military transporter that crashed last month tried in vain to regain control of three engines that froze due to a suspected software problem, according to details of investigations published on Wednesday.

Initial findings on the crash released by Airbus confirmed that the immediate area for concern was the plane’s engine control software, on which it recently ordered safety checks.

It did not say what kind of problem may have caused the engines to freeze but it has ruled out a major structural design issue, focusing attention instead on the quality of final A400M assembly work.

A spokeswoman said investigators were looking mainly at “the different processes around the assembly of the aircraft, the engines and the preparations for first flight”.

The A400M, designed as a troop and heavy cargo carrier for a group of European NATO nations, crashed shortly after takeoff on a test flight on May 9, killing four out of six crew.

The aircraft was being prepared for delivery to Turkey.

Airbus said ‘black box’ data and cockpit recordings indicated that three out of four of the plane’s turboprop engines became stuck at high power shortly after takeoff.

At first, the three affected engines “did not respond to the crew’s attempts to control the power setting in the normal way,” Airbus said in a statement.

Pilots succeeded in reducing power only after setting the thrust levers to ‘idle,’ only to find that the power had frozen once again as they tried to increase it, Airbus added.

Investigators have found that all other systems worked correctly, Airbus said in a bulletin to the plane’s operators.

Spain, where the A400M is assembled, halted production test flights following the crash, effectively halting new deliveries.

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