Thailand’s prime minister vowed to track down those responsible for a blast in central Bangkok on Monday that killed at least 20 people and injured 140. The pledge came as a second explosion hit the city Tuesday, although this time there were no reports of casualties.

Police Senior Sgt. Maj. Worapong Boonthawee told the Associated Press that an explosive device was thrown from a bridge on Tuesday afternoon and blew up at a ferry pier after falling into the river below. He said nobody was killed or injured. No person or group has claimed responsibility for either blast.

Prime Minister Prayuth Chan-ocha said “some suspects” were seen in security-camera footage, but those suspects have not been identified. The prime minister described the incident as the worst attack in the country’s history.

“(In the past) there have been minor bombs or just noise, but this time they aimed for innocent lives. They want to destroy our economy, our tourism,” he said. according to the AP.

He said he believed Facebook messages apparently warning of an imminent danger to Bangkok before the blast came from an “anti-government group” based in the northeast of the country, Agence France-Presse reported.

Monday’s blast, from an improvised explosive device, took place around 7 p.m. local time near the Erawan Shrine, a Hindu religious site popular with Thai Buddhists and Chinese tourists. It is located in a bustling area.

Authorities in Thailand said they are looking for a man who appeared to walk into the shrine Monday wearing a black backpack, and who was then seen shortly afterward without it, the BBC reported. Thai police released surveillance footage of the suspect. The man, who was pictured walking close to the shrine, is wearing a yellow t-shirt and carrying black backpack.

Deputy government spokesman Sansern Kaewkamnerd said that five Thais, two Malaysians, two Chinese mainland nationals, two people from Hong Kong, one Singaporean and eight people of unknown nationality were killed, the AP said.

Thailand’s Defense Minister Prawit Wongsuwan said officials had no prior intelligence about the attack. He said there might be one perpetrator.

“It is much clearer who the bombers are, but I can’t reveal more right now,” Prawit said Tuesday morning, the AP reported. “We haven’t ruled out terrorism.”

“Suddenly there was a big boom, and the whole room just shook, like someone dropped a wrecking ball on top of our ceiling,” Pim Niyomwan, an English instructor who was working in the building next to the shrine told the Bangkok Post. “The whole building just shook. My four students were hysterical.”

On Monday, terrorism experts said the bombing is likely the result of the country’s growing internal strife and is not the work of the Islamic State militants or other international extremists.

Mubin Shaikh, a terror analyst who closely follows Islamic extremists on social media, said there has been no Internet messages that suggest the bombing was the work of such jihadists. “It seems internal,” he said.

Political tensions in the country have increased in recent months. The ruling junta, which came into power last year in a military coup, has made clear that it may not hold elections until 2017 and wants a constitution that will allow some type of emergency rule to take the place of an elected government, the AP reported.

The country has seen recent protests and occasional acts of violence, reflecting the political tensions. Bombings are rare in Bangkok, and terror attacks are more common in southern Thailand, where a Muslim separatist insurgency has been active, the AP said.

Thai shares fell sharply Tuesday following the explosion and the country’s currency, the baht, fell to its lowest level in six years.

Contributing: Jim Michaels