Fourteen of 21 suspects who had been detained as part of an investigation into the Feb. 17 suicide bombing in the Turkish capital were arrested late on Sunday.
Seven others were released, according to judicial sources.
The detainees' courthouse procedures, including a questioning by public prosecutors, lasted for 13 hours, the sources said.
Late on Wednesday a car laden with explosives detonated in Ankara next to military buses near the armed forces' headquarters, parliament and other government buildings, killing 28 people and wounding scores of others.
The Turkish government revealed that the attack was carried out jointly by a Syrian-national YPG member and PKK terrorists based in Turkey. The YPG is the military wing of the PYD, which is the Syrian affiliate of the PKK.
Prime Minister Ahmet Davutoglu said that so far a total of 21 people had been detained in connection with the attack and security measures across the country would be heightened.
A PKK-affiliated terrorist group known as TAK claimed responsibility on Friday for the Ankara attack in a written statement on the internet saying that it would carry out further attacks.
Turkish leaders announced that the PKK and PYD, which is considered by Turkey to be the Syrian arm of the PKK, were behind the deadly Ankara attack.
Turkey has long been confronted with armed attacks in its eastern and southeastern regions by the PKK, which is recognised as a terrorist organisation by Turkey, the US, the EU and NATO.
Davutoglu commented on TAK’s claim of responsibility for the recent bombing, affirming that the group is an extension of the PKK and that the “YPG is also an extension of the PKK. [Therefore] Even though TAK claims responsibility [for the attack], this will not take away the YPG connection.”
YPG is the militant wing of the PYD which operates in northern Syria and is recognised as a terrorist organisation by Ankara due to its connections with the PKK.
However, Washington does not consider the YPG to be a terrorist organisation and is backing the group in the fight against DAESH in Syria.
This has led to growing tension between Ankara and Washington over policy in northern Syria and Davutoglu on Saturday called on the United States to give Turkey its unconditional support in the fight against the YPG.