The U.K. tax authorities have arrested several British bankers on suspicion of using investments in film production to illegally evade tax.

Investigators for H.M. Revenue and Customs arrested four current and one former employee of the Royal Bank of Scotland at their homes last week, along with several employees from two other unnamed banks, according to a report in the Financial Times.

The arrests relate to the bankers’ private financial affairs rather than to any film investment schemes run by the banks themselves. It is not known which film investment schemes are implicated in the probe.

The U.K. government has progressively tightened the rules governing tax relief for film investment over several years, to make it harder for wealthy individuals to use it as a way to avoid tax.

Under current rules, anyone investing more than £25,000 ($39,500) in a film must prove that they spend more than 10 hours a week working on the production to write off the investment against tax.