After nearly 15 months in custody, Iran released Belgian aid worker Olivier Vandecasteele on Friday as part of a prisoner exchange.

In return, an Iranian diplomat convicted of terrorism, Assadollah Assadi, was sent back to Iran. Prime Minister Alexander de Croo of Belgium confirmed Vandecasteele’s release, stating that he had arrived in Oman and would soon return home.

Oman’s foreign ministry announced its role in brokering the exchange deal, facilitating the return of the Iranian diplomat to Tehran.

Iran’s Foreign Minister, Hossein Amir-Abdollahian, expressed gratitude to Oman for its efforts in securing Assadi’s release, referring to him as an innocent diplomat.

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Vandecasteele had been sentenced to 40 years in prison and 74 lashes in January on charges of espionage, which Belgium vehemently denied.

The prisoner exchange was made possible through a treaty signed between Belgium and Iran, which faced criticism for potentially encouraging hostage-taking for negotiation purposes.

While some criticized the release of Assadi, considering it a ransom to terrorism, the Belgian government maintained that it was necessary to secure Vandecasteele’s freedom.

Iran still holds several foreign nationals, regarded as hostages, despite releasing other individuals in recent months.