THE Thai police pledged higher vigilance in crowded areas like bus and train stations after two small blasts rocked downtown Bangkok outside a luxury shopping mall on Sunday night.
The small homemade bombs went off around 8pm along a busy walkway, hurting two people.
Police assistant commissioner-general Prawut Thavornsiri, speaking to reporters on Monday morning, said investigators were still examining footage from closed circuit television cameras and have yet to identify suspects.
He declined to speculate on the cause. "Pipe bombs are easy to assemble and common… any group could be behind it," he said.
Yesterday's blasts were designed to startle rather than to kill, he added.
The bombs were to the first to hit downtown Bangkok after the military imposed martial law and threw out the civilian government in May last year. Prior to that, sporadic violence during the seven months of anti-government protests killed about 30 people.
Since then, political protests and assembly have been prohibited, and many academics, activists and politicians have been placed under strict military surveillance.
While there has been no violent resistance to the coup, human rights activists have decried the loss of civil freedom and criticised the military government's attempts to extend state surveillance through new laws governing digital media.
The coup-maker, Prime Minister Prayuth Chan-ocha, meanwhile stressed that martial law is still necessary to retain peace and order in Thailand.
Political tension was heightened last month when the military-stacked assembly retroactively impeached former prime minister Yingluck Shinawatra, banning her from politics for five years.
Hours before the impeachment, the Office of the Attorney-General announced that she would face charges for negligence.
If convicted, she faces a jail sentence of up to 10 years.
The decision angered her supporters. The military, in response, summoned outspoken former ministers for "attitude adjustment".