Istanbul - As Turkey continues to investigate the Istanbul airport attack and track down suspects, its security forces killed the mastermind of an earlier suicide bombing, an official said on Friday.Mehmet Sirin Kaya was killed in the town of Lice in the mainly Kurdish province of Diyarbakir, the official said. He spoke on condition of anonymity in line with government regulations.The February 17 suicide attack against military personnel in Ankara killed 29 people and was claimed by an offshoot of the Kurdistan Worker's Party, or PKK, which Turkey considers a terrorist organisation.Turkey is still reeling from Tuesday's gun-and-bomb at Atatürk Airport, one of the world's busiest, which killed 44 people and wounded more than 230.Although no group has claimed responsibility, the Islamic State group is the main suspect for that attack. The group has boasted of having cells in Turkey, among other countries.Authorities said the three suicide bombers in the attack - which echoed the carnage earlier this year at the Brussels airport - were from Russia and the Central Asian nations of Uzbekistan and Kyrgyzstan. They did not provide further details on their identities.Thirteen people, suspected of possible links to the attack, were detained in raids in three Istanbul neighbourhoods on Thursday.The ISIS group, which has used Turkey to establish itself in neighbouring Syria and Iraq, has repeatedly threatened Turkey in its propaganda, and the Nato member has blamed ISIS for several major bombings in the past year in both Ankara and Istanbul.Turkey's interior minister said the explosives used were a mix of RDX, TNT and PETN that were "manufactured", which chemist and explosives expert at University of Rhode Island, Jimmie Oxley, described as being military-grade, raising the question of how the attackers obtained the bombs.A key partner in the US-led coalition against the Islamic State group, Turkey also faces security threats from the Kurdish rebels who are demanding greater autonomy in Turkey's southeast region and from ultra-left radicals. Kurdish rebels have carried out numerous car bomb attacks in the past year, including the February 17 Ankara attack and another devastating bombing in the capital in March.