TAK, a faction of the outlawed PKK organisation, claimed responsibility for Saturday's twin bombings in the heart of Istanbul that left 44 people dead.
Who is TAK?
TAK is seen as an offshoot of the PKK, a group which has been waging a violent insurgency in Turkey since 1984. The PKK and its armed wings are listed as terrorist organisations by the European Union, the United States, and Turkey.
TAK said it split from the PKK in 2004. But Turkish authorities see it as an extension of the PKK rather than a separate group, as key PKK figures were involved in its creation.
How are they different from PKK?
TAK said it "split" from the PKK because it had become "weak."
It has carried out several attacks in Turkey, killing dozens of civilians and security personnel.
Global Risks Insights (GRI), a publication which provides analysis on political events around the world, said TAK differs from PKK because of its "willingness to hit civilian soft targets in major urban centres."
The United Kingdom put TAK on its terror list in 2006 and the US followed two years later.
How many attacks have they carried out?
TAK claimed responsibility for Saturday's attack at the Vodafone Arena, two hours after a match between Turkey's top football teams, Besiktas and Bursaspor.
TAK also claimed responsibility for four other major attacks this year.
It was behind two attacks in Ankara – one in February, when 29 people were killed in a suicide bombing, carried out by a woman, that targeted a vehicle transporting military personnel. In March, a suicide attack killed 37 people in Ankara's Guvenpark. TAK said the attacks were in response to Turkey's operation against the PKK.
In April, 13 people were injured In a suicide attack outside a historical mosque in the city of Bursa. A month later a suicide bombing in Istanbul killed 11 people. The attacker was targeting police.