BANGKOK - Thai authorities believe that the person or group behind the bomb blast at downtown Bangkok's crowded Erawan intersection may still be in the city, as they hunt a suspect seen on CCTV footage near the site of the attack that killed at least 21 people and injured over 120.
"We are not ruling out any suspect groups,'' said Professor Panitan Wattanayagorn, a national security specialist and adviser to defence minister and deputy premier General Prawit Wongsuwan.
"The list of suspects is small. They are looking for just a few people. And police chief General Somyot Poompanmoung believes they are still in Bangkok,'' he told The Straits Times.
Prime Minister General Prayut Chan-o-cha was quoted as saying on Tuesday (Aug 18) that the authorities have images of a suspect on CCTV footage planting the bomb.
Some local media reports quoted Thai police sources as saying that the footage shows a "Caucasian'' or ''Arab-looking'' man leaving the bomb under a bench and then walking away, focusing on his mobile phone.
General Prayut said the suspect was believed to be from an "anti-government group based in Thailand's north-east" - a reference to the Red Shirt movement that was subjected to a crackdown by the army after General Prayut - then army chief - seized power in May 2014.
The Red Shirt movement has been involved in violent political unrest of recent years, including grenade attacks, but nothing of the intensity of Monday's bomb in a place chosen to inflict maximum damage to civilians at peak hour.
Some reports said General Prayut's comments suggest that the investigation is shifting towards groups loyal to former premier Thaksin Shinawatra.
But Prof Panitan told The Straits Times that the premier's remark had been misinterpreted.
"Security agencies are not going to make any official claims until they have proper evidence'' he said.
Another speculation is the possible involvement of southern Thai Malay-Muslim separatists, though army chief General Udomdej Sitabutr said on Tuesday that the bomb blast at Erawan "does not match with incidents in southern Thailand".
"The type of bomb used is also not in keeping with the south,'' he said on television.
The insurgency in the south has been harassing and battling Thai security forces on and off for decades, but with particular intensity since early 2004. The main insurgent group, the Barisan Nasional Revolusi (BRN), has attacked civilian targets but not in Bangkok, and very rarely with bombs of the same size as the one in the latest attack. Government spokesman Werachon Sukhondhapatipak said the bomb had 3kg of high explosives.
Given that the Erawan Shrine is popular with tourists from China, another speculation is that the attack could be linked to the Uighurs, a Turkic-speaking Muslim minority in the far west of China who have complained of cultural and religious persecution at the hands of the Beijing authorities.
Last month, more than 100 Uighurs were deported from Thailand to China - a move that prompted widespread condemnation.
But some observers say that while there are violent elements in the Uighur movement, an attack on this scale outside China would be unusual.
Security experts say it is too early to jump to any conclusion over the identity of the perpetrator.
"We have been monitoring them (Uighurs) as well in the past, though the monitoring has been routine and not a top priority'' Prof Panitan said.