Six Ryanair bases in Germany have been raided as part of a probe into claims of systematic evasion of income tax and social security payments involving pilots.

At least 35 prosecutors and customs investigators quizzed pilots and other staff at Ryanair hubs across Germany on Tuesday, including Berlin and Cologne, according to reports.

Investigators seized computers, iPads, rosters and other documents after questioning pilots about their employment relationship with Ryanair.

German prosecutors in the western city of Koblenz reportedly said the raids were part of an investigation into two UK personnel agencies that supply Ryanair with staff, who then work for the airline as self-employed pilots.

The personnel firms are accused of withholding pilots’ salaries and taxes, while pilots are accused of skimping on their social welfare contributions.

Ryanair confirmed in a statement that it had met with and agreed to assist German tax authorities into their investigation of contractor pilots.

“Ryanair requires all of its pilots, both directly employed and contractor, to be fully tax compliant at all times,” said the airline. “The German tax authorities have confirmed that Ryanair is not the subject of any tax investigations.”

However, German investigators said Ryanair’s business model was a central part of the investigation, in particular whether, by not directly employing pilots who fly its aircraft, the airline saves considerable tax and non-wage costs, the Irish Times reported.