Investigators looking into the late November crash of an Air New Zealand Airbus A320 aircraft on a maintenance flight off the southwestern coast of France have been hindered by both a separate French criminal probe and failures to retrieve data from the plane's so-called black boxes, a report in the Wall Street Journal says.

Quoting sources that are “familiar with the details,” the paper’s reporter says “unusually few clues have emerged about possible causes.”

The mystery surrounding the crash – which in good weather without any emergency radio call from the cockpit crew, killing all seven aboard – is troubling aviation safety experts around the world.

This is because the A320 is a workhorse of airlines across the globe and the experts’ desire to whether the crash involved some problem with the plane's flight-control computer.

In the past three months, two larger Airbus models suffered incidents in which flight-control systems malfunctioned, but they didn't result in crashes.

Almost a month after the crash near Perpignan, no usable information has been gleaned from analyzing either the cockpit-voice recorder or flight-date recorder, the sources quoted in the Journal story said.

In one of the earlier A320 incidents, a Qantas Airbus A330 lost altitude after going into a dive while cruising from Singapore to Perth in early October. More than 40 aboard were injured.

Investigators have determined that a problem with part of the plane's computerised flight controls was responsible. European regulators issued a safety directive last week about procedures pilots should use in such circumstances.

The Journal says French and US investigators haven't commented on the causes of the Air NZ Airbus crash. Nor has manufacturer Airbus. Air NZ has only said it is cooperating with investigators.

The A320 was on lease to charter carrier XL Airways Germany, and was flown by two XL Airways pilots on a maintenance flight. The plane had undergone maintenance at EAS Industries, a company in Perpignan that overhauls and repairs Airbus and Boeing jetliners.

The plane had been repainted in Air NZ livery and was slated to be returned to the carrier following the flight. In addition to the pilots, four Air NZ employees and a New Zealand civil aviation official died in the crash.

The Journal report says when initial efforts failed to retrieve data from the plane's recorders, French air-crash investigators decided to send them off to Honeywell international, which made the devices.

But a criminal probe of the crash has delayed that transfer by a number of weeks, according to people familiar with the details. At this point, Honeywell experts don't expect to receive the recorders until the first week in January. A Honeywell spokeswoman confirmed to the Journal that the recorders had not been received.

Earlier this month, Flight International magazine quoted the French air-accidents bureau saying that "at this stage of the investigation, nothing can explain why the aircraft went off course and crashed into the sea."