If, as the evidence suggests, Monday's attack was a car bomb, the list of potential suspects is quite long.

Earlier this month, a violent Turkish leftist group claimed responsibility for a suicide attack on the U.S. Embassy in Ankara, killing a Turkish guard and seriously wounding a Turkish journalist.

For nearly three decades, Kurdish separatists have fought a guerrilla war against the Turkish state.

And al Qaeda and other radical Islamic groups have carried out deadly attacks in Turkey.

The opposition Syrian National Council has accused the Syrian government of planning Monday's explosion, but has not offered any evidence to back up that claim.

On Tuesday, Turkish gendarme units established a checkpoint blocking civilian access to the Cilvegozu border terminal.

And in the nearby town of Reyhanli, families were burying victims of the mysterious blast.

Hundreds of mourners watched as the wife and young son of 34-year-old Ahmet Tas, a Turkish citizen killed in the attack, screamed in despair.

His coffin was buried in a cemetery not far from the Syrian border.