A suspect was Tuesday detained by police in connection with six explosions at ATMs in four districts of Pattani.
A police source said the suspect, whose name was withheld, was arrested after he was captured on closed-circuit television.
The footage showed he was one of six men who arrived in the Pattani municipality area before separating to plant bombs at six ATMs in four districts.
The police source said he could not disclose more details about the suspect as that might affect the probe.
He said police are still collecting evidence and questioning witnesses and plan to seek court approval to arrest the remaining five suspects soon.
The source said an initial investigation had found the culprits were both former and current members of the Barisan Revolusi Nasional (BRN), which is a Pattani independence movement.
The investigation found that some of them were involved in explosion cases in the deep South in 2013 and the court had issued arrest warrants for them at the time, he said.
The explosions on Sunday evening injured two people. In Muang district of Pattani alone, three bombs went off with one of them wounding two women.
Sompit Sakoljit, 54, and Somjit Srisombat were injured in a blast at an ATM in front of Kasikornbank on Yarang Road in Pattani's Muang district.
One bomb planted at an ATM at a hotel was made safe before it could go off while another suspicious-looking package turned out to be a false alarm. The other explosions took place in Nong Chik, Yarang and Sai Buri districts.
The source said that the eight explosions in Yala were likely the work of Masukree Yusoh, a close aid of the late Sapa-ing Basor, a former headmaster of the Thammawithaya Foundation School in Yala and a key BRN leader.
Mr Masukree has been accused of carrying out attacks in Yala in the past.
Authorities believe he carried out bomb attacks in Betong and was involved in the torching of a double-decker interprovincial bus in Bannang Sata district last year, the source said.
Of the eight Yala explosions, authorities reported two grenade attacks in four districts of Kabang, Bannang Sata, Than To, and Raman. There were no reports of any injuries.
An investigation is under way to arrest suspects involved in two blasts and a gun attack in Rangae and Bacho districts of Narathiwat as well as in Songkhla where two explosions were reported, one occurring at an ATM located at a petrol station in tambon Sakom, Thepha district.
The other was at an ATM in tambon Ban Nod, Saba Yoi district. The cash dispensers were damaged but there were no reports of injuries.
More than 20 incidents were reported in these four provinces on Sunday evening.
Some turned out to be false alarms, while two bombs were defused.