There was no immediate claim of responsibility for the latest carnage, which reduced cars and buses to charred husks on a busy road in the heart of the city last evening, wounding more than 120 people.
But Ankara believes one of the bombers was a woman with ties to the outlawed Kurdistan Workers' Party (PKK), a Turkish official told AFP today.
Turkish police detained four people near the Syrian frontier today, state-run Anatolia news agency reported, acting on a tip-off that the car used in the bombing had been bought in Sanliurfa, a Kurdish-dominated town some 50 kilometres (30 miles) from the border.
The fact that extremists were able to strike again in the heart of the capital, so close to many sensitive buildings and so soon after February's attack will raise questions about Turkey's ability to deal with the twin threat of Kurdish rebels and the Islamic State (IS) group.
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Hours after the attack, Turkish fighter bombers hit PKK arms depots and shelters in mountainous northern Iraq, the army said, quoted by Anatolia.
Health Minister Mehmet Muezzinoglu today gave a new toll of 37 from yesterday's blast targeting a bus stop, but said this included at least one attacker and possibly two. The first funerals for the victims were held today.
Yesterday's attack bore similarities to another suicide car bombing on a convoy of military buses which killed 29 people in Ankara on February 17.
That attack was claimed by the Kurdistan Freedom Falcons (TAK), linked to the PKK, as revenge for Turkish military operations in the southeast. The TAK warned of more attacks to come, including on tourist areas.
The PKK, classed as a terrorist group by Ankara and its Western allies, launched a bloody insurgency in 1984 demanding an independent state for Kurds.