Volkswagen is skidding further into the emissions scandal ditch. The German carmaker has already displayed startling ineptitude, waiting to admit wrongdoing until after an independent investigation found that it rigged 500,000 vehicles in the United States and 11 million cars globally to cheat on pollution tests. Now American watchdogs may have uncovered even more misconduct.

Europe's largest car manufacturer has been adamant that it installed so-called defeat devices only on a certain type of diesel engine of up to 2 litres in capacity. But the Environmental Protection Agency and the California Air Resources Board on Monday accused it of putting the gadgets on bigger engines, too.

In a carefully worded statement, Volkswagen said it "wishes to emphasize that no software has been installed in the 3-litre V6 diesel power units to alter emissions characteristics in a forbidden manner." If the latest charges are true, though, they would shred the company's official defence.

What's more, they would call into question the company's internal investigation, currently overseen by new Chairman and former long-time finance chief Hans Dieter Poetsch. Volkswagen has already raised doubts about its efforts by insisting the probe is still in the early stages and essentially blaming the scandal on a small group of engineers.

One of the company's first steps after news of the scandal broke in September should have been to root out any other vehicles that risked raising regulatory suspicions - and letting watchdogs know. The financial considerations alone are compelling: The EPA can slap Volkswagen with up to $18 billion in fines for the misconduct identified initially. Cooperating with the government is one way to cut that amount substantially. On the other hand, any indication that the company has been less than forthcoming or lax in its investigation could be extraordinarily costly.

The revelation that Porsche may be involved in the mess creates another dangerous twist. Matthias Mueller ran Porsche before becoming the overall boss of Volkswagen after the scandal first hit. The prospect of him now being dragged into the mire is enough to give the company's board nightmares.

(The authors are Reuters Breakingviews columnists. The opinions expressed are their own)

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