Seven students were arrested yesterday by the Thai police following a series of bomb blasts in Bangkok on Friday, but they admitted to planting only one explosive device to attack their rivals, police said.
The teenagers told investigators they had no political motives or links to the other five explosions that occurred in other parts of Bangkok on the same day, said police chief Teerapong Wongratpitak of Metropolitan Police Division 4 yesterday.
The boys, aged 15 to 17, are students at a technical school.
They confessed to placing the device on Rama 9 Road in Suan Luang district that injured three street cleaners.
Apart from the Rama 9 explosion, two other small blasts went off near the BTS Skytrain Chong Nonsi station in central Bangkok, and three at a government complex in Chaengwattana Road on the outskirts of Bangkok.
A seventh unexploded device was later recovered by the authorities.
Police have not made any other arrests so far.
The security scare added to the two fake bombs found by the police on Thursday near the Centara Grand and Bangkok Convention Centre, where the 52nd Asean Foreign Ministers' Meeting and related gatherings were being held. The regional meetings attended by senior global leaders ended on Friday.
Two men have been arrested in connection with the fake-bomb incident.
Meanwhile, a fire - suspected to be a case of arson - broke out in the commercial district Siam Square on Friday. A manhunt has been launched for a man acting suspiciously as seen on a store's surveillance footage.
Prime Minister Prayut Chan-o-cha has ordered a probe into the explosions and the other incidents, which could be personally embarrassing to him as he hosted the high-profile meetings after being elected premier in June by Parliament amid controversy, following the March general election.
No group has claimed responsibility for the other five blasts.
Mr Prayut said at a news conference on Friday that an "old group" could be the possible culprits behind the explosions.
He was referring to his political rivals - the Red Shirt group - comprising supporters of former prime ministers Thaksin and Yingluck Shinawatra.
"But I am not ruling out other possibilities," Mr Prayut added. "Whoever did this, they want to destroy the country. They shouldn't have a place in this country."
But national police chief Chakthip Chaijinda indicated on Friday that initial investigations pointed to a group of southern Muslim insurgents as the perpetrators.
Still, the police also did not rule out that the explosions may be backed by political and other groups.
The Muslim insurgents were widely blamed for a series of bombings in seven southern and central provinces in August 2016 that left four people dead and dozens injured.
Moving to restore confidence among foreign visitors, Tourism and Sports Minister Phiphat Ratchakitprakarn and officials from his ministry visited Bangkok's Chinatown on Friday evening, shaking hands with many tourists and giving them assurances that the Thai capital is safe.
Mr Phiphat later told the press he believed the incidents would not affect Thailand's booming tourism industry, which saw a record 38.2 million visitors last year.