Chechen Akhmed Chatayev was the mastermind of the 2016 Istanbul airport attack, according to Turkish authorities.

Georgian security services say they have killed the suspected mastermind of the 2016 bombing of Istanbul’s Ataturk Airport.

Akhmed Chatayev was killed in a special operation in Tbilisi, the capital of Georgia, during which three other people also died. The 20-hour standoff ended with three suspects killed and one arrested, while one officer was killed and four wounded.

The identities of the other two slain suspects remained unclear. “The investigation is ongoing. We continue to work with our international partners to identify the other two,” Nino Giorgobiani, the deputy chief of Georgia’s state security service, told the Reuters news agency.

Georgian authorities are looking into what the suspects were doing in Georgia and how they got there.

Chatayev, a 37-year-old from Chechnya, was believed to have been a member of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group. Interpol says that Chatayev was seen as one of the more senior Russian-speaking ISIL members, responsible for training and commanding at least 130 fighters.

He lost his arm during the Second Chechen War, which took place between 1999 and 2009. In 2015, he moved to ISIL-controlled territory and was placed on a US Treasury sanctions list for planning attacks against US and Turkish facilities. He was also put on a UN sanctions list, wanted by Russian authorities for “terrorist crimes committed in its territory”, according to Interpol.

Last year, Chatayev was named by Turkish and American intelligence services as the mastermind behind the 2016 bombing at Ataturk Airport in the Turkish city of Istanbul.

Three suspects fired automatic weapons and detonated suicide vests on June 28, 2016, killing 45 people and injuring more than 200.