FRANKFURT (Reuters) - Public prosecutors on Tuesday raided offices of German exchange operator Deutsche Boerse DB1Gn.DE as part of a broader investigation into a fraudulent share-trading scheme, Cologne public prosecutor's office said.

A Deutsche Boerse spokesman confirmed that offices of its Clearsteam subsidiary were searched, adding that the investigation was focused on both clients and staff.

“As in the past, Deutsche Boerse is fully cooperating with the authorities,” the spokesman said.

Handelsblatt first reported the raids, adding that Clearstream apparently played a key role in the illegal dealings.

The investigations focus on a practice, known as cum-ex, which involves cross-border trading of company shares around a syndicate of banks, investors and hedge funds to create the impression of numerous owners, with each entitled to a tax rebate.

The investigation into the scheme began as far back as 2013. Prosecutors have said the practice misled the German government into paying tax refunds that have cost the country billions of euros.