The Turkish government says Kurdish militants appear to be responsible for twin bomb attacks on police outside a stadium in Istanbul on Saturday.
Deputy PM Numan Kurtulmus said initial findings pointed towards the Kurdistan Workers’ Party (PKK), which has targeted security forces in the past.
At least 38 people died near Istanbul’s Vodafone Arena when attackers struck after a top-division football match.
A car bomb hit a police vehicle and a suicide bomber blew himself up nearby.
Mr Numan told CNN Turk news channel that about 300-400kg (660-880lb) of explosives had been used in Saturday evening’s attack.
“Arrows point” to the Kurdistan Worker’s Party (PKK), he added. No group has said it carried out the bombings.
Attacks in Turkey this year have been carried out both by the PKK and so-called Islamic State (IS).
Turkey is a member of the US-led coalition fighting IS. It is also facing a renewed conflict with the banned PKK in south-eastern Turkey.
President Erdogan told reporters that Turkey would fight “the scourge of terrorism to the end”, and promised that the attackers would pay a “heavy price”.
The site of the attack reopened swiftly, with floral tributes laid on the ground and people bearing Turkish flags. The government is as always keen to give a sense that the situation is under control.
But beneath the surface, Turkey feels vulnerable and afraid that it can’t stop the wave of bombings. The government has vowed revenge and President Erdogan says terrorism will be crushed.
But perhaps the most tumultuous year in Turkey’s history means many will see the words as empty rhetoric. This is, though, a defiant nation. One protester told me terrorists “aimed to keep us at home, scared of going out. We can’t do that. We must show unity against this threat”.
After bombings by the PKK and IS and an attempted coup, Turkey finishes 2016 angry, grieving and even more politically polarised. A toxic mix for a country that has seemingly lost its way.
Interior Minister Suleyman Soylu said 13 people had been arrested after the latest blasts, but gave no details.
Thirty of those killed were police officers. Dozens of people remain in hospital, some in intensive care.
said the explosions had “aimed to maximise casualties”.
But they came two hours after the end of the match between top-division teams Bursaspor and Besiktas at the Vodafone Arena in central Istanbul.
Local media reported that fans had already dispersed. Bursaspor posted on Twitter that it knew of no injuries to its supporters.