Airat Vakhitov, a Tatar formerly imprisoned at the U.S. military facility in Guantánamo Bay, Cuba, for ties to the Taliban, has been arrested in Turkey in connection with last month’s siege of Istanbul Atatürk Airport.

Members of Vakhitov’s current jihadist group, the Association of the Russian-Speaking Mujahedeen in Turkey, confirmed to Voice of America that he had been among the 30 people arrested in connection with the Istanbul attack. Salman Sever, a spokesman for the group, asserted that Vakhitov had “never fought in Syria or Iraq, he was never part of IS [the Islamic State/ISIS], [and] he was involved in humanitarian activities for the Syrian people.”

Vakhitov was detained in Afghanistan in a Taliban raid in 2001 and spent two years in Guantánamo before being handed to Russian authorities, Turkey’s Hurriyet newspaper notes. The Islamic State in its current form did not exist at the time of his arrest; it was still an offshoot of al-Qaeda. Since his release, he has been working with alleged humanitarian groups after renouncing his Russian citizenship and allegedly “seeking refuge in the Middle East.” He has been a featured speaker at an Amnesty International event, condemning the practices at Guantánamo Bay as “torture” but describing them as milder tactics than those he experienced in Russian detention.

Voice of America asserts that Vakhitov had spent much of his recent years in Istanbul, noting that they could not immediately find information regarding his current citizenship after renouncing his Russian identity.

A 2015 review of former Guantánamo Bay prisoners at Fox News described Vakhitov as a Tajik national who had been “accused of using a local human rights group as cover for his activities.”

On June 28, three attackers believed to be associated with the Islamic State stormed Istanbul Atatürk airport, armed with firearms and suicide vests. The men shot into the crowd at the international terminal, shouting, “Allahu akbar” and detonated their vests, killing over 40 people.

Turkish authorities identified the three jihadists as Russian, Uzbek, and Kyrgyz nationals, blaming “Caucasian” Islamists for the spread of Islamic State ideology and terror. At least two of the attackers possessed Russian passports and are believed to have trained in neighboring Syria.

Vakhitov is the latest in a string of Guantánamo Bay prisoners believed to have returned to jihad. In February, Spanish authorities announced that they had arrested an unnamed former Guantánamo detainee in the territory of Ceuta, planning terrorist acts in the country with a number of other jihadists in the region.

Late last year, Ibrahim Al Qosi, an al-Qaeda affiliate, appeared in a propaganda video for the terrorist group, threatening the United States and encouraging Muslims to wage “war” against America.

In March, Paul Lewis, a Pentagon special envoy tasked with closing the Guantánamo facility, told the House of Representatives that there is proof that Guantánamo alum have killed Americans after being released. “What I can tell you is unfortunately there have been Americans that have died because of [Guantánamo] detainees,” he admitted. “When anybody dies, it is tragedy and we don’t want anybody to die because we transfer detainees.”

At least one former detainee, Jihad Ahmed Dhiab, has gone missing; it is unknown whether he is currently pursuing involvement in Islamic terrorist activities once again. Dhiab left Uruguay last month, allegedly to travel to Brazil. Brazilian authorities have confirmed they have no record of him entering the country, and his whereabouts are unknown. Dhiab has disappeared a month before the 2016 Summer Olympics in Rio de Janeiro, a prime target for jihadist attacks.