Seven more suspects have been arrested in connection with last Friday's bombings in Bangkok and Nonthaburi, government spokeswoman Naruemon Pinyosinwat said on Tuesday.
The new arrests bring the total number of suspects in custody to nine, said Ms Naruemon.
She said the National Security Council (NSC) has reported that investigators were questioning all the suspects, and Prime Minister Prayut Chan-o-cha is being provided with daily updates on the progress of the investigation.
The first two suspects, both from Rueso district of Narathiwat, were apprehended on an inter-provincial bus at a police checkpoint in Chumphon province last Friday.
A security camera had captured images of one of the two men dropping a suspicious-looking object near a 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.
Deputy Prime Minister Prawit Wongsuwon said on Tuesday that the bombers probably came from southern border areas, but investigators had yet to establish whether the attacks were politically motivated.
A source at the Royal Thai Police said that evidence showed that a total of 15 suspects were involved in the bomb and arson attacks in the capital.
The source said that four suspects were involved in the arson attack in Soi 19 off Phetchaburi Road in the Pratunam area.
They used a power bank and alcohol to carry out the attack, the source, said. The four had travelled to Bangkok on July 20 and stayed in the Ramkhamhaeng area before carrying out the attack, the source said.
Seven bombs were reported in five locations in Bangkok and Nonthaburi last Friday, with five of them going off, hurting five people.
The blasts occurred at Chong Nonsi BTS station near the King Power Mahanakhon building, the Government Complex on Chaeng Watthana Road, and the Royal Thai Armed Forces Headquarters.
Army chief Gen Apirat Kongsompong said on Tuesday the masterminds of the Bangkok bombings last week were people who want to discredit the government.
"They wanted to undermine confidence at a time when the country was hosting meetings of foreign leaders. We have seen such incidents during the previous Asean meeting in Pattaya in 2009," Gen Apirat said.
"The same old group that was the mastermind [back in 2009] wants to create all kinds of unrest," Gen Apirat claimed.
