The prosecutor's office in the city of Braunschweig announced that the investigation would focus on "allegations of fraud in the sale of cars with manipulated emissions data."

"The aim of the investigation is to clarify the chain of responsibility," it added.

Winterkorn stepped down last Wednesday after US authorities revealed that Volkswagen had cheated in emissions tests using software installed in diesel vehicles. The software recognized when a vehicle was being tested and reduced the amount of nitrogen oxide emitted, although under road conditions the cars belched out up to 40 times as much of the ozone-causing substance as US law allows.

The scandal has spread as other countries announced they would launch probes into the diesel engines involved.

The presidium of VW's supervisory board last week announced it would itself file charges against persons unknown over "obvious irregularities" with regard to diesel engine pollution tests. Board members said they would fully support investigators in their work.

Spreading ripples

Shares in Volkswagen continued their downward slide in mid-day trade on Monday, shedding nearly 7 percent as the scandal over rigged pollution tests showed no sign of abating.

According to German media reports, VW had ignored warnings from staff and supplier Bosch years ago that there was an illegal emissions-test-rigging software in use in a large number of vehicles.

Audi said Monday that 2.1 million of its cars worldwide were fitted with the so-called defeat devices that allowed parent company VW to thwart emissions tests in the US.

Audi noted that 577,000 of those units were sold in Germany and almost 13,000 in the US, and added that a wide range of models were affected, including A1, A3, A6, TT, Q3 and Q5 cars.

Heads rolling

Last Friday, VW's supervisory board named Porsche chief Matthias Müller to replace ousted CEO Martin Winterkorn and indicated that more executive heads would need to roll to make a fresh start at the Wolfsburg-based company.

Citing sources familiar with the matter, Reuters reported Monday that Volkswagen had suspended the R&D chiefs of its core brand (VW), luxury division Audi and sports-car maker Porsche until the scandal was fully cleared up.

German authorities heaped pressure on the embattled carmaker, demanding it set out a timeline by October 7 as to how it would ensure its diesel cars met national emissions standards without using the cheat technology.

hg/jd,tj (AFP, Reuters)