The Turkish authorities said one of several groups could be behind the twin suicide bombings in Ankara that killed 97 people on Saturday. Prime Minister Ahmet Davutoglu said investigations were focused on the Islamic State in Iraq and Syria (ISIS) as the prime suspect.
Here is an overview of the groups Mr Davutoglu cited as possible suspects, even as a Turkish opposition party put forward another theory.
ISIS: The government blamed ISIS for a suicide bombing on July 20 in the town of Suruc, on the Syrian border, that left 33 people dead. ISIS has seized swathes of neighbouring Iraq and Syria, and Turkey was long criticised by its Nato partners for not doing enough to crack down on the militant group. But now, Turkey has fully joined the United States-led coalition in the fight against ISIS, and is allowing US warplanes to use its key southern base at Incirlik. This has raised fears that Turkey could be vulnerable to revenge attacks by the extremist group.
NTV television said initial investigations pointed to a connection to ISIS and the suspected remnants of the suicide bombers would be subjected to DNA testing.
It added that experts believed that the type of bomb used was very similar to the Suruc attack, and the same Turkish forensic team used in Suruc was now working in Ankara. The Hurriyet and Haberturk dailies reported that the elder brother of Abdurrahman Alagoz, who carried out the Suruc suicide bombing, may be implicated in the Ankara blasts. Meanwhile, Turkish police have arrested 43 suspected members of ISIS in raids across the country, in cities such as Adana and Antalya.
KURDISTAN WORKERS' PARTY (PKK): The Kurdish militant PKK blamed the government after the Suruc attack, and resumed attacks on the security forces after a two-year truce. According to state media, more than 140 members of the security forces have been killed in the violence. Pro-government media, such as the Sabah daily, have implied a PKK hand in the Ankara attacks.
However, Kurdish activists were among those present at the peace rally that was targeted. Also, the PKK, on the same day as the attack, announced a de facto ceasefire, saying it would launch attacks only in self-defence.
REVOLUTIONARY PEOPLE'S LIBERATION PARTY-FRONT (DHKP-C): The DHKP-C has claimed responsibility for a string of attacks in Turkey in recent months, including a gun attack on the US embassy in Istanbul, but most have been small in scale and, sometimes, even amateurish in nature.
The DHKP-C, known until the mid-1990s as Devrimci Sol (Revolutionary Left), seeks to foment a Marxist revolution among the working classes but also espouses a fiercely anti-Western and anti-Nato agenda. It claimed responsibility for taking prosecutor Mehmet Selim Kiraz hostage on March 31 at his Istanbul office. The crisis ended with the killing of the hostage and both hostage-takers during a police raid.
'MAFIA STATE': The leader of the pro-Kurdish Peoples' Democratic Party (HDP), Mr Selahattin Demirtas, blamed a "mafia state", and a "state mentality that acts like a serial killer" for the attack.
With tensions high, ahead of Nov 1 snap elections, the HDP fears the government will try to benefit at the ballot box from the unrest. Many Turks are fearful of acts that the secret security services - known as the "deep state" (derin devlet in Turkish) - might commit.
However, the government angrily ridiculed the notion of any state involvement.