As part of an investigation into cheating software, the German authorities invaded Tuesday at Volkswagen's headquarters in Frankfurt, the automaker confirms.

Volkswagen came under global scrutiny in 2015 after it was discovered in the United States that diesel engines could cheat on emissions testing using software. Through the software, the engines seemed to emit a lot less than was actually the case.

This so-called tampering software was for cars with diesel engines of the EA 189 type. The investigation of the German authorities now focuses on its successor: the EA 288 type. In 2015, Volkswagen said that the EA 288 did not include tamper software.

Volkswagen says in a press release that it cooperates with the German authorities, but adds that the accusations of the prosecutor and the investigation are unfounded. According to the automaker, the diesel engine is not tampered with.

The research focuses on "individuals," Volkswagen writes. What the functions of these persons are and why they are important in the investigation is unknown.

The 2015 diesel scandal cost the German automaker a total of around 30 billion euros in settlements, fines and repairs.

See also: You need to know about Volkswagen's diesel scandal