France, Dec. 4 -- On Tuesday, Belgian authorities arrested the trio after searching the EU diplomatic service's headquarters in Brussels and the College of Europe, a prestigious training institute for future policymakers, in Bruges.

The European Public Prosecutor's Office (EPPO) said on Wednesday that Mogherini, now rector of the College of Europe, was among those detained, along with a senior college staff member and an official from the European Commission.

The EPPO said the Belgian Federal Judicial Police questioned all three before formally notifying them of the accusations. These relate to suspected procurement fraud and corruption, conflict of interest and breaches of professional secrecy.

All three were released afterwards, with...