The Thai authorities are looking for a man seen on CCTV footage leaving his bag at Bangkok's popular Erawan Shrine and walking away just before Monday's deadly explosion as another blast yesterday sent fresh jitters across the city.
The nationality of the suspect, who took a commuter train and a tuk-tuk to the area, is not clear.
Experts say the operation was well-planned and that the man could not have acted alone. No one has claimed responsibility so far.
The powerful rush-hour blast killed 20 people, 12 of them instantly. Several foreigners, including Singaporean Melisa Liu Rui Chun, 34, were among the dead. Her husband Ng Su Teck, 35, was among seven Singaporeans who were injured.
Yesterday afternoon, someone threw a bomb that exploded at a pier on the Chao Phraya River, next to the Shangri-La hotel. It was badly aimed and bounced into the water before going off.
A nearby vendor said there was a loud bang, followed by water shooting up metres into the air. People at the scene fled.
Police chief Somyot Poompanmoung, who went to the scene, said the bomb was similar to Monday's.
"This is aimed at discrediting the government and harming tourism," General Somyot said.
The pier was closed and divers deployed to retrieve remnants of the bomb for examination.
Speaking on TV yesterday afternoon, Prime Minister and former army chief Prayut Chan-o-cha said: "It is apparent that there are active individuals or groups that harbour the intention to damage Thailand."
They may be "pursuing political gain or other intentions by damaging the economy", he added.
He urged media outlets to report "news that will be constructive". Reports that expound on violence and destruction "may have a misleading effect on ongoing investigations and create misunderstandings in our society", he added.
Singapore leaders expressed shock and condemned that attack.
Foreign Minister K. Shanmugam, speaking in Parliament, said: "We strongly condemn this heinous attack. Nothing can justify the killing of innocent civilians."
Deputy Prime Minister Teo Chee Hean also expressed his condolences on Facebook, stressing the need to stay vigilant.
Mr Teo, who is also Coordinating Minister for National Security and Home Affairs Minister, urged people to do their part in reporting suspicious activities or persons, adding that securitymeasures here have been stepped up.
Thai Tourism Minister Kobkarn Wattanavrangkul told The Straits Times a 200 million baht (S$8 million) fund, augmented by 6 million baht from the private sector, would be used to compensate blast victims. Her ministry would give 300,000 baht each to the families of tourists killed on Monday and 100,000 baht each to the injured. It would also pay for the relatives' travel and transport expenses as well as hotel accommodation, she said.
The bomb blast would definitely impact the tourism industry, she added, though it is too early to say to what extent.
Analyst Ambika Ahuja of Eurasia Group told The Straits Times: "The impact on tourism will likely be more extensive and longer-lasting than past incidents, given the extent of the damage and the number of foreign tourists among the casualties."
Tourism, which accounts for about 10 per cent of Thailand's growth output, has been the only bright spot recently for an economy hampered by political uncertainty and sluggish consumption.
The Ratchaprasong intersection opened to vehicles late yesterday after a clean-up operation. Though the shrine, which was not damaged, remains closed, a small group of Thais lit candles and held a vigil there at 7pm, the time of the blast.
- Additional reporting by Tan Hui Yee in Bangkok