The Muslim-majority border region has seethed with violence for over a decade as ethnic Malay insurgents battle the Buddhist-majority state for more autonomy.
Bombings and shooting are a weekly occurrence.
But yesterday two bombs -- a small device followed by a much larger car bomb -- went off outside a busy supermarket in the town of Pattani, the largest attack for months on a civilian target in the south.
Colonel Pramote Prom-in, spokesman for the southern army, said the investigation was focused on two people who allegedly rode up on a motorbike to drop off the first device and on two others who left the much bigger car bomb outside the supermarket.
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Authorities said more than 60 people were wounded in the blast, some of them seriously, with 20 victims still in hospital today.
Ethnic Malay militants have been fighting the military in Thailand's three southernmost reasons since 2004 after dozens of civilians died in army custody.
Some 6,800 people have since died in the conflict, most of them civilians, with both sides accused of rights abuses.
The region was annexed by Thailand more than a century ago and has long simmered with resentment over Bangkok's push to assimilate the ethnically and linguistically distinct locals.
Soldiers, police and local officials are frequent victims of the attacks but are also known to pursue soft civilian targets including teachers and monks.
Rights groups and UNICEF -- the UN's children's agency -- condemned yesterday's attack as one that was clearly aimed at killing or maiming large numbers of civilians.
Amnesty today described the blast as "a horrific and deliberate attack on civilians" that showed "a callous disregard for human life".
Don Pathan, an expert on the southern insurgency, described Tuesday's blast as "very strange" because it so clearly targeted civilians, many of whom were local Muslims.
The simmering insurgency plays out far from Thailand's popular tourist resorts and receives little international attention as a result.
Unsuccessful peace talks have staggered on for years with little success.
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