The authorities in Belgium are looking into whether Spain’s security services installed a GPS tracker in the car of Carles Puigdemont to follow the deposed Catalan president’s movements while he was in the country. The car in question is the same vehicle Puigdemont was traveling in when he was detained by German police on his return to Belgium from Helsinki.

The Brussels attorney general’s office confirmed the existence of an investigation on Thursday. During a news conference in the Belgian capital, a spokeswoman for the public prosecutor said “an investigation is ongoing” but she gave no further details, except to say that it is being carried out by the prosecutor’s office in the Walloon Brabant province, which is where Puigdemont was living.

Puigdemont was detained by German police on Sunday March 25 at a motorway service station near the Danish border. The president was returning to Belgium from Finland, where had attended a conference. At the time, German police confirmed that Puigdemont’s arrest was made possible by information supplied by Spain’s intelligence service, the CNI.