ISTANBUL 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, bringing deadly attacks to its biggest cities in violence fueled by the wars in Syria and Iraq.
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 luxury BMW 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 maximize the damage, the sources said. 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.
"The use of moving vehicles that are highly destructive in suicide attacks is a new phenomenon in Turkey," said Metin Gurcan, an independent security analyst and retired Turkish military officer who now writes a column for Al-Monitor.
"These kind of attacks are seen in Syria and Iraq as part of daily life but Turkey is becoming the kind of place where they occur too," he said. "It is not new that the PKK is targeting civilians ... but the scale is new."
A 2-1/2 year truce in Turkey's southeast broke down in July after the PKK's political arm announced an end to the ceasefire, saying the state was using the lull in fighting to build new military outposts and prepare for war.
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 centers, turning parts of the region into a war zone.
Ankara sees the spiraling violence as deeply linked to events in 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.
"In many instances ... the same terrorists, the same militants, go to Turkey with a PKK flag and when they pass to the Syrian side they put on the YPG flag. It is the same thing," President Tayyip Erdogan's spokesman Ibrahim Kalin told reporters in Ankara last month.
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 destabilize Turkey, something the YPG's political arm has strongly denied.
"(The PKK) has obtained large amounts of explosives after ammunition and weapons depots in Syria and Iraq were looted," said Mete Yarar, a security analyst and former Turkish special forces officer who was in the security forces for 20 years.
"These are usually military-type explosives which you can't buy on the open market."
Erdogan and government ministers have said attacks such as those in Ankara will not lessen Turkey's resolve in fighting terrorism, signaling continued air strikes on PKK camps in northern Iraq, military ground operations in southeastern Turkey and potentially renewed shelling of the YPG in Syria.
But these are blunt tools which leave many in Turkey questioning whether their government has the capacity to prevent further attacks. "We ask who will protect the people's lives," the opposition Sozcu newspaper said in its front-page headline the morning after the Ankara bombing.
Security experts say such attacks, using moving suicide car bombs, are extremely difficult to prevent, particularly with the chaos in Syria and Iraq hampering intelligence gathering.
"For an intelligence agency to work properly, you need a counterpart on the other side. That creates a system where you can filter people and threats," said Yarar. "But there is none near Turkey's 1,200-km southeastern border."
Turkey is also hampered in its ability to strike the YPG in Syria because the group has enjoyed U.S. support in the fight against Islamic State, much to Ankara's frustration, which views it as an extension of the PKK.
Ultimately, most observers believe there can be no military solution, making a return to a peace process with the PKK initiated by Erdogan in 2012 seem almost inevitable. But that could be some time coming, with the government insisting it can only resume once the militant group lays down its weapons.
"It is a known fact to both sides that this conflict can not be resolved militarily," said Nigar Goksel, senior analyst for Turkey at the International Crisis Group. "We think in the end, both sides will be coming back to the negotiating table, but they are both trying to come back more powerful."
OUT OF CONTROL
Part of the problem is the questionable extent to which the Turkish state's past negotiating partners are still in control. A new generation of Kurdish militants, many still teenagers, has taken center stage in the recent violence and some want to take their armed campaign outside the southeast.
"Inside Turkey there is a lot of anger among Kurds. Hundreds of people have been killed, cities have been destroyed, and there has always been this underlying question of why is the PKK fighting in areas where Kurds get hurt," said Aliza Marcus, author of "Blood and Belief," a history of the PKK.
The Ankara bombing "points to a new and really dangerous shift in the war between the PKK and the Turkish state. The PKK is showing it is willing to take its war into (Turkey's) west, that it wants to make Turks suffer the way Kurds are suffering."
The previous suicide bombing in Ankara on Feb. 17, which targeted a military bus and killed 29 mostly soldiers, was claimed two days later by the Kurdistan Freedom Hawks (TAK), which says it split from the PKK.
Security experts say the two retain close links.
TAK is the PKK's urban, western wing and offers the latter, which is concerned about its international standing, "plausible deniability" while implementing its strategy, Marcus said.
"The PKK provides TAK with logistical, training and weapons support but gives TAK the initiative to make decisions including where, when and how to carry out attacks," said Gurcan.
"Unfortunately, this region produces a lot of youth who will die easily for ethnic or religious motives. University students and 14-year-olds who want to die."
(Additional reporting by Orhan Coskun in Ankara, Nick Tattersall in Istanbul; writing by Nick Tattersall; editing by Philippa Fletcher)