Under police interrogation, one of two foreigners in detention admitted to delivering a rucksack to the bomber at Bangkok’s main railway station on the evening of the August 17 attack, two police sources told Reuters.
Police believe the bomber is the yellow-shirted man still at large and caught by surveillance camera placing a backpack at the Erawan Shrine moments before the explosion that killed 20 people and wounded 130.
“The suspect insisted he didn’t know what was in the bag,” one of the sources involved in the investigation said.
The man being questioned, according to another police source, talked of a lead role played by a man named “Izan”, who had fled via Bangkok’s main airport a day before the attack. He had assigned responsibilities during a meeting of those involved.
“From the testimony we found that Izan is likely to be the mastermind but has already left,” he said, without specifying his nationality or destination.
“It is likely others we’ve issued arrest warrants for have already fled.”
The three police sources declined to be identified because they are not authorised to speak to media.
The details, which have yet to be confirmed, would mark rare progress in a case which the authorities have been criticised for handling haphazardly.
Deputy police chief Chakthip Chaijinda told Reuters the man had confessed to being part of a network and to being in the area of the bombing. Chakthip did not elaborate, but added police were looking for four more suspects.
Evidence has been limited to grainy security video at the shrine and the seizure of bomb-making materials in raids on two properties in Bangkok.
Wearing a flak jacket, the man Chakthip said had confessed was made to perform a reconstruction at the two locations before television cameras on Tuesday.
Forensic tests tie the detainees to the explosives, but not the site of the bombing, police say.
Authorities said there was no clear motive for the attack, in which 14 foreign tourists were killed.
Speculation has been rife about the perpetrators, from ethnic Malay insurgents and foreign militants to opponents of the government and sympathisers of Uighur Muslims from China, angered by Thailand’s deportation to China in July of 109 members of the Turkic-speaking minority.