A Porsche Cayman is seen in a Porsche shop at Siam Paragon mall in Bangkok, Thailand June 4, 2017. REUTERS/Jorge Silva - RTX38YWD

FRANKFURT (Reuters) - Prosecutors in Stuttgart have launched a preliminary probe against employees at German sportscar maker Porsche AG to see whether they were involved in designing engine management software to cheat emissions tests.

The Stuttgart probe is the latest part of a sweeping investigation of Volkswagen Group VOWG_p.DE, which owns the Audi, VW and Porsche brands. Illegal software has been found in VW, Audi and Porsche cars equipped with diesel engines.

Prosecutor Jan Holzner said on Thursday the Porsche probe was not a formal investigation but was rather still at a preliminary stage.

Porsche had no immediate comment.

Some Porsche models are equipped with 3 liter diesel engines supplied by Audi. Prosecutors in Munich are separately probing Audi about its role in designing the 3 liter diesel engine.

Stuttgart prosecutor's office is also formally investigating three employees at automotive supplier Bosch [ROBG.UL] for their possible role in aiding and abetting emissions fraud by Volkswagen AG VOWG_p.DE, Holzner said.

“We are investigating three employees. All three are managers, with the highest ranking being in middle management,” he said, adding he could not rule out that the probe would be widened.

Bosch helped to develop an engine control unit, known as EDC17, for Volkswagen’s EA189 diesel engine which later failed emissions tests.

Bosch said it takes allegations of engine manipulation “very seriously” and is fully cooperating with authorities. Because of the ongoing nature of the investigation Bosch declined to comment on details.