Authorities in Thailand Tuesday released an image of a man they believe to have carried out Monday's bombing that killed at least 20 people and injured 140 during Bangkok's evening rush hour.  

The image, apparently taken from closed circuit surveillance video, shows a man in a yellow T-shirt carrying a backpack. Video footage posted on Thai media websites appeared to show the same man sitting on a bench at the crowded Erawan Shrine, then taking off the backpack and leaving it behind as he walked away.

Police Lt. Gen. Prawut Thavornsiri had earlier released several photos of the man, with and without the backpack, on a social media platform. When contacted by The Associated Press, Prawut said, "The yellow shirt guy is not just the suspect. He is the bomber."

Sky News quoted Thavornsiri as telling the Thai news website Khaosod that the suspect "travelled alone. He arrived at the [shrine] by tuk-tuk. Then he left the crime scene by a motorcycle taxi."

"The yellow shirt guy is not just the suspect. He is the bomber."

- Lt. Gen. Prawut Thavornsiri

Nerves in Bangkok were further frayed Tuesday afternoon when another explosive device blew up at a ferry pier -- this time harmlessly. Police Senior Sgt. Maj. Worapong Boonthawee said an explosive device was thrown from the Taksin Bridge and blew up at Sathorn Pier after falling into the Chao Phraya River below. "There is no injury," he said. Security camera footage shows a sudden blast of water over a walkway at the pier as bystanders run for safety.

Tuesday morning, Prime Minister Prayuth Chan-ocha called the attack "the worst incident that has ever happened in Thailand," and he promised to track down those responsible.

"There have been minor bombs or just noise, but this time they aimed for innocent lives," Prayuth said. "They want to destroy our economy, our tourism."

Without elaborating about possible perpetrators, the prime minister said Tuesday, "Today we have seen the closed-circuit footage, we saw some suspects, but it wasn't clear. We have to find them first."

The improvised explosive device scattered body parts, spattered blood, blasted windows and burned motorbikes to the metal. The explosion went off around 7 p.m. (8 a.m. ET) in an upscale area filled with tourists, office workers and shoppers. No one has claimed responsibility.

Bangkok was rattled again Tuesday when another explosive device blew up at a ferry pier often used by tourists, but no one was hurt.

"The bomb at Sathorn Pier was also a pipe bomb and it might be related to the (Monday) bombing," said Prawut, the police spokesman. Another police official, Senior Sgt. Maj. Worapong Boonthawee, said it was thrown from the Taksin Bridge and blew up at Sathorn Pier after falling into the Chao Phraya River below. "There is no injury," he said. Security camera video showed a sudden blast of water over a walkway at the pier as bystanders run for safety.

Prayuth gave his first televised address since the bombing Tuesday, saying the government will expedite "all investigative efforts to find the perpetrators and bring them to justice."

Thai authorities identified five victims as Thai and four as Chinese -- two of them from Hong Kong -- along with two Malaysians and one Singaporean, and said the nationalities of the other eight victims remained unknown.

Defense Minister Prawit Wongsuwan said authorities had no idea an attack had been planned.

"We didn't know about this ahead of time. We had no intelligence on this attack," the defense minister said.

Prayuth vowed to "hurry and find the bombers," though he noted there may be just one perpetrator. Speaking to reporters, he continued what has been a notoriously prickly relationship with the media since the former general took control of the government in a May 2014 coup.

Asked if there were leads on the suspects' identities, Prayuth bristled, "We are still investigating. The bomb has just exploded -- why are you asking now? Do you understand the word investigation? It's not like they claim responsibility."

Thailand has seen many violent attacks in recent years, particularly through a more-than-decade-long insurgency by Muslim separatists that has left more than 5,000 dead in the country's deep south. Those attacks have never extended to the capital, however.

Bangkok has seen politically charged violence over the past decade; the deadliest, in 2010, killed more than 90 over two months and was centered on the same intersection where Monday's bomb went off. But none of those attacks included a bomb blast that seemed intended to produce mass casualties.

Matthew Wheeler, Southeast Asia security analyst for the International Crisis Group, said the bombing was a "new type of attack for Bangkok" that doesn't bear the trademarks of typical violence over the past decade from political instability or Muslim separatists.

"It is certainly not like politically motivated attacks we've seen in the past which have generally been designed to grab attention but not cause casualties," Wheeler said, adding that he expected it would have "major ramifications for security in Thailand."

At the scene, investigators surveyed the damage as police and soldiers guarded the area, still littered with shattered glass and other debris. The normally busy intersection that was closed off to traffic and eerily empty aside from onlookers standing behind police tape to take pictures. Barricades were set up outside five-star hotels in the neighborhood and security stopped cars to inspect trunks before letting them pass. The intersection was reopened by midday.

National chief of police Somyot Poompanmoung said the bomb was made with a pipe wrapped in cloth and weighed more than 6 pounds.

The shrine is dedicated to the Hindu god Brahma, but is extremely popular among Thailand's Buddhists as well as Chinese tourists. Although Thailand is predominantly Buddhist, it has enormous Hindu influence on its religious practices and language.

Bangkok has been relatively peaceful since a military coup ousted a civilian government in May last year after several months of sometimes violent political protests against the previous government.

At the same time, the military government has tightly controlled dissent, arresting hundreds of its opponents and banning protests. Tensions have risen in recent months, with the junta making 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.

Stirring the pot has been exiled former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 coup. It was his sister Yingluck Shinawatra who was ousted as prime minister last year.

Last week, Thaksin posted a message on YouTube urging his followers to reject the draft constitution because he said it was undemocratic. The draft charter is supposed to be voted on next month by a special National Reform Council. If it passes, it is supposed to go to a public referendum around January.

The U.S. Embassy in Bangkok issued an emergency message for U.S. citizens, advising them to avoid the shrine's area. In Washington, State Department spokesman John Kirby expressed deep sympathy to those affected by the Bangkok explosion. He said authorities were still determining whether any Americans were among the victims.

Tourists reacted with concern.

"We didn't think anything like this could happen in Bangkok," said Holger Siegle, a German who said he and his newlywed wife had chosen Thailand because it seemed safe. "Our honeymoon and our vacation will go on, but with a very unsafe feeling."

The Associated Press contributed to this report.