No one has claimed responsibility for the bombing spree, which hit seven southern provinces on Thursday and Friday and left dozens wounded, including European tourists.
Police said they know who to blame but have yet to reveal a culprit or suspected motive.
They have ruled out international terrorist groups, calling the attacks an act of "local sabotage".
"Our investigation is progressing. We know who was behind it," deputy national police spokesman Piyapan Pingmuang told AFP, declining to give further details.
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"It is likely they were intended to explode at the same time of the previous explosions," said Grisada Boonrach.
Police said other unexploded devices were found yesterday in Phang Nga province, which had been struck by bomb blasts and a suspected arson attack the day before.
One man has been arrested over a separate case of arson in Nakhon Si Thammarat province and two men have been held for questioning over the Hua Hin bombings, police said.
"It's just asking questions. They will not be treated as suspects unless the questioning procedure is done and any of them are found to have violated laws," said Colonel Winthai Suvaree.
Some analysts suggest the assault was the work of Muslim rebels waging a long-running insurgency in Thailand's southern tip, but Thai officials have rejected that theory.
If the rebels are to blame, it would mark an unprecedented escalation of a 12-year revolt so far confined largely to the border region.
The kingdom has been battered by a decade of political unrest, driven by a bitter power struggle between the military-allied elite and populist forces loyal to the ousted democratically elected government.
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