Thai investigators believe a man seen in security video wearing a yellow T-shirt and carrying a backpack set off the bomb at a central Bangkok shrine that killed 20 people and injured more than 100.
"The yellow shirt guy is not just the suspect. He is the bomber," police spokesman Lt Gen Prawut Thavornsiri said.
Prawut earlier released several photos of the man, with and without the backpack, on social media.
The images were apparently taken from closed-circuit video at the Erawan Shrine on Monday evening before the bomb exploded. He confirmed that the man is suspected in the bombing.
A video posted separately on Thai media appeared to show the same man, with shaggy dark hair, sitting on a bench at the crowded shrine, then taking off the backpack and leaving it behind as he walked away.
Prime Minister Prayuth Chan-ocha called Monday's explosion at a busy intersection "the worst incident that has ever happened in Thailand," and promised to track down those responsible.
"There have been minor bombs or just noise, but this time they aimed for innocent lives."
Without elaborating about possible perpetrators, he added: "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, which police say was made from a pipe and weighed three kilograms, scattered body parts, spattered blood, blasted windows and burned motorbikes to the metal.
The explosion went off around 7pm in an upscale area filled with tourists, office workers and shoppers. No one has claimed responsibility.
Bangkok was rattled again yesterday 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 said that 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.
Monday's bomb exploded at Erawan Shrine, which is dedicated to the Hindu god Brahma, but is extremely popular among Thailand's Buddhists as well as Chinese tourists.
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.
The British Foreign Office said one victim was a British citizen named Vivian Chan who lived in Hong Kong.
It was not immediately clear whether she was one of the two Hong Kong victims.