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Prosecutors in Stuttgart said they are investigating whether Robert Bosch GmbH employees helped Volkswagen AG rig software to cheat on emissions tests.

The probe is looking at corporate staff at the car-parts maker, which provided VW with the software, but no suspects have been identified, Claudia Krauth, spokeswoman for Stuttgart prosecutors, said in an interview Tuesday.

"In the wake of what was reported about the emissions case at Volkswagen, we concluded that someone must have produced the software," said Krauth. "It was obvious which company came into question."

The probe puts a spotlight on Bosch’s role in the emissions scandal at Volkswagen, which disclosed in September that it cheated on pollution trials with its diesel cars. The software, which was installed in some 11 million vehicles worldwide, lowered the level of nitrogen oxides emitted only when it detected the car was being tested.

Bosch spokesman Rene Ziegler said the Stuttgart-based company is cooperating with the authorities.

Bosch supplied two components linked to exhaust treatment that are “standard” in many cars, a company spokesman said in September. Volkswagen was responsible for how parts were applied and integrated, he said at the time.

Bosch produces diesel engine parts for automakers including diesel emission treatment systems such as SCR, or selective catalytic reduction. Bosch warned Volkswagen in a letter in 2007 that the way the carmaker planned to use the software installed in diesel engines was illegal, Bild-Zeitung reported on Sept. 27, without saying where it got information.

Grappling with the fallout from the revelations, VW set aside 6.7 billion euros ($7.4 billion) in the third quarter for diesel recall costs, not including potential regulatory fines and damages from hundreds of lawsuits.