The people responsible for the grenade explosion outside the Criminal Court on Saturday night are linked to those behind the two small blasts outside the Siam Paragon mall last month, the national police chief said yesterday.
Police General Somyot Poompanmoung said both groups of perpetrators were "in the same movement" but had different methods in carrying out their attacks.
He said the police investigators would summon any individual mentioned by the suspects arrested earlier if a connection is established. These include former armed forces supreme commander General Chaisit Shinawatra, who is linked to the Pheu Thai Party, and former Metropolitan Police chief Lt-General Kamronwit Thoopkrajank.
"If there is a connection with the people mentioned by the suspects, we will question all of them, without an exception," Somyot said.
Both Chaisit and Kamronwit yesterday denied any connection with those arrested for throwing a grenade at the court.
Chaisit said some people tried to link him to the latest bomb attack because his name and phone number appeared in a notebook of one of the arrested suspects. He said that could be because of his family name.
But he said he would co-operate with police in their investigation into the case. "I am ready to give the information I have. And I will not escape," Chaisit said.
He also threatened anyone who causes damage to his reputation linking him to the grenade attack.
Kamronwit also denied any involvement with the perpetrators, saying he had not been involved with politics since leaving the force. He said he had just learnt that his name and contact number appeared in a notebook of one of the arrested suspects.
Meanwhile, red-shirt leader Suporn Atthawong went to the Royal Thai Police headquarters yesterday to file a police record that his volunteer defence group, which was dissolved shortly after the coup, had no involvement with the arrested suspects.
He noted that the group's white scarf was found among the suspects' belongings.
Two men have been held in connection with the grenade attack. They were arrested after a brief shoot-out with security forces. The attack caused minor damage to the court building car park but no injuries.
Arrest warrants have been issued for another man and two women from the same apparent cell. Police said the suspects were linked to the red shirts, major supporters of Pheu Thai Party.
Red shirt leader Jatuporn Prompan dismissed the link with his group yesterday, insisting they had nothing to gain from stoking tensions under martial law.
"We are not involved. It would be a disadvantage to us ... no-one is that stupid," he said on his television programme.
The Metropolitan Police yesterday appointed 38 senior police to be part of a working group responsible for investigating the grenade attack, which is the second one since the coup.
The team will be headed by Metropolitan Police deputy commissioner Maj-General Jitti Rodbangyang.
Meanwhile, Prime Minister Prayut Chan-o-cha said yesterday he thought the person who hired the perpetrators to carry out the attack was even more stupid than the bombers.
He called on members of the public to be the "eyes and ears" of security forces in order to boost international confidence in Thailand after the second explosion in the capital in just over a month. "We ask the public to be the eyes and ears of authorities because security must come first. We must rebuild confidence for foreigners," he said.