The two suicide bombers, identified as Musa Canöz, 28, and Adnan Yıldırım, 40, were allegedly scouting the capital's central Kızılay square and nearby bars, the location of many New Year's Eve celebrations, for two separate attacks.
The bombers took order from “Ebu Enes” who gave information over the Internet one day before the planned attack to “conclude preparations, finalize explorations, wait tomorrow until the action and conduct attacks simultaneously in two separate positions.”
Police also detained Z.A. who had provided transport and had been in contact with Canöz and Yıldırım.
The two left Ankara in 2013 to join IS and re-entered Turkey in September 2015 through Kilis with the help of smugglers before traveling to Ankara by bus and renting an apartment with cash.
Canöz and Yıldırım had received training on explosives in Syria and prepared the bomb set-ups which were also seized by police during a raid on their location. They purchased chemicals for explosives in the Çubuk district of Ankara and other materials in the Ulus district.
The decision for the attack was taken in Raqqa, IS's capital, with bombers acting independently of ISIL cells already in Turkey.
Canöz and Yıldırım confessed that they would carry out an attack on New Year's Eve. Canöz stated that he returned Turkey to take revenge for his brother, who was killed during clashes with the People's Defense Units (YPG) in Kobane in northern Syria.
After the move to detain Canöz and Yıldırım, Ankara police intensified security measures in Ankara's Kızılay Square on Dec. 31, 2015, and allowed people to enter the area only after body searches.
The security measures were at the same level as those taken during most political rallies. The square was emptied before the celebrations to allow police dogs to search for explosives.
Some 982 people from Ankara, including women and children, have joined IS since 2012, 19 of whom were killed in clashes.