Turkish authorities on Tuesday charged the prime suspect in the twin car bombings that left 51 people dead in a town near the Syrian border, the state-run Anatolia news agency reported.
The man, who was arrested trying to cross into Syria along with two accomplices, is alleged to have purchased the vehicles used in the bombings. Four other suspects were also charged.
Turkish police have arrested dozens of people over the attack and 12 people have been charged so far, Anatolia said.
All suspects are Turkish nationals who Ankara believes were backed by the Syrian regime.
Damascus has denied any involvement and has offered to carry out a joint investigation, which was firmly rejected by the Turkish government.
The 11 May attacks were Turkey's deadliest case of a spillover of the Syrian conflict, which started in March 2011 with peaceful protests against the rule of President Bashar al-Assad and has now degenerated into a civil war that has killed tens of thousands of people.
Turkey, a onetime ally and today a vocal critic of the Assad regime, currently hosts some 400,000 refugees from its neighbour, as well as rebels and army defectors.