The Belgian aid worker exchanged with an Iranian terrorist last week wrote a letter to thank all those who mobilized his release.

Olivier Vandecasteele said that after 15 months of arbitrary detention he was finally released and is doing fine.

“Having had no direct access to outside information for the entire duration of my detention in solitary confinement (i.e., 13 consecutive months), I was miles away from imagining the extent of the citizen mobilization - which all of you have worn day after day - across the country and across borders,” read his letter, published on Twitter.

He also thanked the efforts of the Belgian government while calling on governments to continue efforts to release further hostages held around the world.

“Let us bring active support to projects that carry the values of humanity, solidarity, and hope. This world, which is unfortunately often terribly cynical, needs it now more than ever,” he wrote.

Activists were outraged over an Oman-brokered swap deal according to which Iranian diplomat Assadollah Asadi who was convicted of terrorism, was freed in exchange for Vandecasteele.

Assadi, a former attaché at the Iranian embassy in Austria who was convicted of plotting to bomb a gathering of the exiled opposition group Mujahedin-e Khalq Organization (MEK) near Paris on June 30, 2018, was released and arrived in Tehran on Friday.

Vandecasteele, who was detained last year and sentenced to 40 years in prison and 74 lashes for alleged “spying and cooperation with the United States, money laundering and smuggling $500,000 out of Iran,” was also released as part of the deal.