A seven-month independent probe into the Volkswagen exhaust emissions affair has failed to find those responsible for the scandal that has engulfed the German carmaker, officials close to the matter told dpa on Friday.

The US law firm Jones Day was appointed by the VW supervisory board last October to draw up a report into the scandal, which emerged last September when Europe's biggest carmaker admitted that it had cheated on emissions tests on about 11 million diesel models around the world.

But while Jones Day has narrowed the origins of the software used to manipulate the exhaust tests to a group of departments and individuals, it has been unable to reconstruct the chain of events that led to the development of the scam, the sources said.

The investigation's failure to unearth the source of the illegal manipulation of tests was despite around 450 investigators searching through a massive database representing more than 102 terabytes of information, or more than 40 times the size of the Panama Papers release.

The lawyers have, however, ruled out any involvement in the scandal by the former VW group chief executive Martin Winterkorn, who stood down from his post last September as the crisis erupted. Winterkorn was replaced as VW chief by former Porsche head Matthias Mueller.

A company spokesman declined to comment on the lawyers' report, which is likely to leave open key questions about the scandal that has battered the company's global image.

In particular, the question remains whether individual VW developers took on it on themselves to develop the so-called defeat device or whether they were acting on instructions from senior management.

VW said on Friday that global sales of its core Golf and Passat brands slumped by 2.7 per cent in March compared with the same period last year, resulting in first-quarter deliveries falling by 1.3 per cent year on year.

The drop was even more pronounced in the US, where the emissions scandal first emerged. First-quarter VW core brand sales plunged by 12.5 per cent compared with the corresponding period in 2015.

VW is facing a critical phase in the scandal as it battles the mounting costs of lawsuits, official investigations, recalls and vehicle modifications as well as the fall in sales around the world in the wake of the revelations about the emissions affair.

Based in the northern German city of Wolfsburg, the company is also facing the launch later this month of tough union pay talks with union leaders calling for a pact to ensure job protection.

VW indicated in November that it is planning cuts in its workforce after it announced moves to cap investment spending at 12 billion euros (14 billion dollars) in 2016 - a cut of 1 billion euros from recent years.

In addition, VW must meet a deadline next week to produce its plan for modifying about 580,000 US vehicles to return their emissions to within legal limits. The deadline has been set by the US judge overseeing hundreds of civil suits against the carmaker.

The auto group's 20-member supervisory board is also likely to try to hammer out at a meeting next Friday a compromise in a battle over bonus payments for top managers, which could involve executives agreeing to a 30 per cent cut in benefits.

The German state of Lower Saxony, one of VW's major shareholders, has joined the board's union representative in calling for bonuses to be scrapped as the carmaker deals with the costs of the emissions scandal.

The following week the company will unveil its 2015 annual results, which are likely to reveal the impact of the crisis on the carmaker's finances.

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