Police have dismissed reports that the bombings in Bangkok were in revenge for the death of a suspected insurgent sympathiser who died while in detention at a military camp in the deep South.
Deputy spokesman Krissana Pattanacharoen denied the reports and said that Prime Minister Prayut Chan-o-cha has instructed national police chief Chakthip Chaijinda to step up efforts to track down and arrest those involved.
According to earlier media reports, two southern men arrested in connection with a bomb found at the Royal Thai Police (RTP) headquarters claimed they and eight accomplices wanted to create chaos in Bangkok to avenge the death of a suspected sympathiser detained by the Fourth Army Region at a military camp in the South.
The two men, both from Rueso district of Narathiwat, were apprehended on an inter-provincial bus at a police checkpoint in Chumphon province on Friday.
A security camera had captured images of a man dropping a suspicious looking object near the fence outside RTP headquarters on Rama I Road on Thursday evening.
The object was found to be a time bomb set to go off at 8am on Friday, but it was defused by bomb disposal officers.
Another source on the investigation team said security camera footage found another four suspects allegedly involved in the bombings.
The four men boarded an inter-provincial bus from Songkhla's Hat Yai district on July 31 and got off at Mo Chit bus terminal in Bangkok the following day.
The four then took a taxi to Makro's Rangsit branch in Pathum Thani province where they changed their clothes before splitting into two groups of two, each group taking separate taxis.
One headed to the Government Complex on Chaeng Watthana Road in Bangkok and the other went to the Office of the Permanent Secretary for Defence in Nonthaburi's Pak Kret district.
They then took taxis to Mo Chit, where they took a bus back to Hat Yai district on Thursday evening. Bombs went off at the two locations on Friday morning, the source said.
The source added the four suspects spoke Yawee, the Malay dialect spoken by locals in the deep South. Police are gathering evidence to seek arrest warrants for the suspects.
Pol Gen Chakthip has issued an order to set up an investigation team to handle the bombing case. The team comprises 16 police generals led by deputy national police chief Srivara Ransibrahmanakul.
Police reported on Saturday that seven bombs were reported at five locations in Bangkok and Nonthaburi on Friday.
The incidents occurred while the capital was hosting a high-profile gathering of foreign ministers from Asean and 21 other countries, including the US, China and Russia.
The blasts included one at the Chong Nonsi BTS station near the King Power MahaNakhon tower, two at the Government Complex on Chaeng Watthana Road, and at the Royal Thai Armed Forces headquarters.
Police on Friday night arrested seven students on suspicion of planting bombs in connection with an explosion in Soi 57/1 off Rama IX Road, but the students denied any involvement with the blast.