Turkish officials say the two suicide bombers responsible for nearly 100 deaths at an Ankara peace rally last week had fought in Syria and had connections with Islamic State.
The Hurriyet newspaper said the bombers were identified as Yunus Emre Alagoz and Omer Deniz Dundar, both from the conservative mixed Kurdish-Turkish city of Adiyaman in Turkey's southeast.
Alagoz was the brother of the Islamic State suicide bomber who attacked a gathering of Kurdish activists in the frontier town of Suruc in July, killing more than 30 people who were planning to move reconstruction supplies into the battered city of Kobani, Syria. Islamic State laid siege to Kobani for nearly six months beginning late last year before mostly Kurdish fighters drove the militants out.
The two men reportedly traveled in separate cars from the city of Gaziantep, a key logistics and resupply point for Islamic State militants and other rebel factions, to Ankara.
"I went to the police and told them, 'Put him in jail,'" Dundar's father said in an interview with Hurriyet. "They questioned and then released him and he went back to Syria."
A third man accused of acting as a scout in last week's bloody attack has been arrested, along with the owners of the cars used in the bombing, Turkish news reports said.
The father's statements prompted outrage in Turkey, and top security chiefs have been removed from their posts as part of the fallout from what was the nation's deadliest terrorist attack.
"In order to run a healthy investigation of the abominable terrorist attack ... and in line with the requests from chief civil and police inspectors, Ankara's provincial police chief, intelligence department chief and security department chiefs have been removed from duty," the Interior Ministry said in a statement late Tuesday.
Turkish President Recep Tayyip Erdogan acknowledged a security lapse, in his first public comments since the attack, which prompted massive public outrage and grief.
"There must undoubtedly be a mistake, a shortcoming in some place," said Erdogan, who laid bouquets of flowers at the blast site on Wednesday. "Of what dimension? This will emerge after examinations."
Johnson is a special correspondent.