Austrian police say they have arrested 24 suspects in an investigation of a Serbia-based drug ring that allegedly rented houses in and around Vienna to grow marijuana.

The federal criminal police office said the arrests came during raids of 14 grow sites in Vienna and surrounding Lower Austria province Friday. The raids were part of an international investigation launched in late 2017.

Police said Sunday that the suspects rented the houses with forged documents and turned them into indoor farms.

They think the group produced at least 888 kilos (1,958 pounds) of marijuana since February 2017 and sold it for an average of 3,900 euros per kilo, or over 3.4 million euros (nearly $4 million) in all.

Two suspects were already in custody. Of those arrested, 24 are Serbian.