BANGKOK (AFP) - Thai police discovered unexploded bombs in three top tourist destinations over the weekend as they searched for clues to a wave of blasts that rocked resort areas and killed four last week.
No one has claimed responsibility for the bombing spree, which hit seven southern provinces on Thursday (Aug 11) 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.
Unexploded devices were found and defused on Sunday (Aug 14) in two bomb-hit areas - the upscale beach town of Hua Hin and the popular island of Phuket, according to the interior ministry permanent secretary.
"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 on Saturday 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.
A junta spokesman confirmed that multiple people have been questioned but stressed it was too early to identify them as suspects.
"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 blasts are seen as an affront to a military government that prides itself on having brought some stability to Thailand since its 2014 coup.
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.
But the violence has not matched the coordinated nature of the latest bombings or targeted tourist towns.
The attacks came only days after the junta won a referendum vote on a controversial new charter it drafted.
The document, which critics say will make Thailand less democratic, was approved by 61 per cent of voters but rejected in the north and north-east - strongholds of the ousted government - and in the three insurgency-torn southern provinces.
The shadowy southern rebellion has left more than 6,500 people dead since it erupted in 2004.
But the violence rarely makes international headlines or affects Thais outside the conflict zone, a Muslim-majority region annexed more than a century ago.
Analysts say the rebels are frustrated over stalled negotiations with the military government.
"It looks like the work of (the insurgents), judging from their kind of arms... it was not aimed to create mass casualties, so that's very similar to the far south," Don Pathan, a security analyst and expert on the insurgency, said of last week's bombings.
But he said the junta would be loath to admit a major expansion of the conflict, since it would signal a significant "policy failure in the south".
A leader of the "Red Shirts" - the grassroots movement supportive of the ousted government and hostile to the junta - expressed concern on Sunday his network would be fingered for the attacks.
"We have been made victims for things we did not do several times before," Jatuporn Prompan said in a video posted on Facebook.
The Red Shirts, who hail chiefly from the poor and rural north-east, are fiercely loyal to the powerful Shinawatra family, whose repeated election victories have been undone by two coups and a series of judicial rulings in the past decade.
Their political network has come under heavy surveillance by the military since the 2014 coup.
The bombings in top tourist destinations threaten a vital source of income for tropical Thailand.
The sector accounts for at least 10 per cent of an economy the military government has struggled to revive.