Two doctors at the center of the investigation into the circumstances surrounding the death of Greek pop star Giorgos Mazonakis on Wednesday were led to Athens Police Headquarters on Thursday evening.

The development came after the completion of a lengthy search of the doctors’ clinic in Kolonaki, which was carried out by officers and authorities in the presence of a prosecutor.

Police seized a computer, folders containing documents, and various substances from the premises, all of which are expected to be sent for laboratory analysis. Two plasmapheresis machines were also found at the clinic. According to the Athens Medical Association, the installation of such machines is not permitted in a clinic.

Health Minister Adonis Georgiadis said on Wednesday night that the clinic was operating under a license for a general medical practice and was therefore not authorized to conduct plasmapheresis, a procedure that filters certain components from the blood.

Mazonakis had either undergone, was undergoing, or was preparing to undergo such a procedure.