The second suicide bombing in a month in the heart of Turkey's capital has raised fears of a dangerous new phase in its war with Kurdish militants.
Security officials say the two perpetrators of Sunday's car bombing, a man and a woman, were linked to the outlawed Kurdistan Workers Party (PKK), which has waged a three-decade insurgency for Kurdish autonomy in southeast Turkey.
Their car was driven from the southeastern town of Viransehir, and the explosives it carried when it targeted a crowded transport hub in central Ankara were laced with pellets and nails to maximise the damage.
Thirty-seven people were killed, and dozens more wounded.
Prime Minister Ahmet Davutoglu said Turkey had obtained "very serious and almost certain" evidence suggesting the PKK was responsible but there has been no claim of responsibility.
Turkey faces numerous security threats, including from Islamic State militants, and government critics say it is too quick to blame Kurdish insurgents.
The PKK has largely avoided civilian targets in the recent past.
Should links to the PKK or its affiliates be confirmed, the mass targeting of civilians and relative sophistication of the attack would mark a deadly change in tactics from a group that had previously concentrated for the most part on striking the security forces in the southeast.
A two-and-a-half-year truce in Turkey's southeast broke down in July after the PKK's political arm announced an end to the ceasefire.
Since then the violence has been at its worst since the 1990s, with hundreds killed as the security forces try to drive out militants who have dug trenches and erected barricades in urban centres, turning parts of the region into a war zone.
Ankara sees the spiralling violence as deeply linked to northern Syria, where the PKK-affiliated Kurdish militia, the Peoples' Protection Units (YPG), have seized territory near the Turkish border as it battles Islamic State militants and rebels fighting President Bashar al-Assad.
Turkish security officials say border crossings controlled on the Syrian side by the YPG are being used as a conduit for supplies, ammunition and recruits to destabilise Turkey, something the YPG's political arm has strongly denied.
Turkey is also hampered in its ability to strike the YPG in Syria because the group has enjoyed US support in the fight against Islamic State.
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