Police suspect locals and a student at an Islamic school colluded with an insurgent group to launch the first-ever bomb attacks in Satun and Phatthalung over the weekend.
Three residents have been questioned by security investigators, but they denied any involvement, Satun police chief Suphawat Thapkliao said on Tuesday.
Satun was struck by six bomb blasts, including one in the compound of a police station, late on Saturday night in Muang district. No one was hurt in the blasts.
Investigators are continuing to review security camera footage for more clues about suspects they believe might have "cooperated" in the attacks, Pol Maj Gen Suphawat said.
Eleven bombs also exploded in the early hours on Sunday in Phatthalung's Muang and Pak Phayun districts. There was no report of any casualties.
Police investigators are turning their attention to suspects who may still be in hiding in the two districts where nearly 30 Islamic schools are located.
Many students from the nearby southernmost provinces attend classes there, and at least one student from one of the Islamic schools in Pak Payun district may have had a role in the attacks, according to police who are collecting evidence ahead of a potential arrest.
Insurgent groups, which are usually active in the Muslim-dominated provinces of Pattani, Yala and Narathiwat, may have expanded their areas of attack beyond the southern border provinces, which would make it difficult for the government to prevent further violence, according to a state intelligence agency.