BERLIN—German prosecutors have launched an investigation into three employees of Volkswagen AG’s luxury car unit Audi suspected of falsifying documents to obtain roadworthiness certifications needed for vehicles to be exported to South Korea, a Munich prosecutor said on Monday.

“We have three suspects, but there could be more,” Karin Jung, a Munich prosecutor, told The Wall Street Journal. The three are suspected of manipulating test results and mileage readings of the vehicles and falsifying the serial numbers, the prosecutor said.

This is one of several probes of Audi stemming from the 2015 admission by Volkswagen that it rigged millions of diesel-powered vehicles to cheat on emissions tests, although it isn’t directly related to the emissions cheating. Former Audi CEO, Rupert Stadler, ousted by Volkswagen last month, has been in jail since June on allegations that he tried to tamper with witnesses in the diesel probe.

Audi, one of Volkswagen’s main profit engines, also faces strong headwinds in its European business in the wake of a new emissions-testing regime. Audi’s European sales fell 56% in September and are down 7% so far this year.

Audi and some other European auto makers have suffered delays in certifying vehicles under the emissions testing rules that took effect in September, causing a drop in sales. German auto makers have also been hit by falling demand for diesel vehicles.