German police on Friday morning detained Enrique Mora, the top EU official who has been coordinating talks on reviving the Iran nuclear deal, in Frankfurt airport as he was traveling en route to Brussels from Iran.

Mora, who holds a Spanish diplomatic passport and has been the EU’s pointman for the Iran negotiations, was detained for 30 minutes, an EU official confirmed to POLITICO.

“Retained by the German police at the Francfort airport on my way to Brussels, back from Teheran. Not a single explanation,” Mora tweeted on Friday morning. “An EU official on an official mission holding a Spanish diplomatic passport. Took out my passport and my phones.”

Mora, a deputy secretary-general for the European External Action Service (EEAS), the EU’s foreign policy wing, had been in Tehran as negotiations intensify on reviving the nuclear deal, with the United States close to returning to the pact. Former U.S. President Donald Trump in 2018 walked out of the agreement signed by his predecessor Barack Obama in 2015.

In a tweet sent later on Friday morning, Mora confirmed he had been released, along with the EU ambassador to the United Nations in Vienna and the head of the EEAS Iran task force. “We were kept separated,” Mora said. “Refusal to give any explanation for what seems a violation of the Vienna Convention.”

The convention covers diplomatic relations and grants immunity to those who hold diplomatic passports. It also states that diplomatic passport-holders should not be hindered in their travels.

POLITICO has contacted the German foreign office and police for comment.

Peter Stano, a spokesperson for the EEAS, said: “There is nothing to comment on, the issue was solved quickly and Mr Mora continues with his onward travel as planned. We are in touch with German authorities and will not comment further.”

In a tweet, Mora also noted that he had raised the need to stop the execution of Iranian-Swedish scientist Ahmadreza Djalali while he was in Tehran, asking for his release on humanitarian grounds.

Iran has said it will execute Djalali, accusing him of spying for Israel and involvement in the assassination of Iranian scientists. Djalali has denied all the charges against him.

Mora, who is 63, is a former Spanish diplomat who has been coordinating talks with Iran over reviving the Joint Comprehensive Plan of Action, the official name of the landmark nuclear deal.

This story has been updated.

Jacopo Barigazzi contributed to this report.