Videos taken by nearby CCTV and tourists show how the blast hit a crowded shrine in Thailand's largest city.
Police are now hunting the yellow-shirted man seen dumping a backpack at the site of the blast.
In the new footage, he is shown carefully and deliberately removing his backpack inside the shrine, getting up without it and immediately leaving the scene.
The spot where he was sitting is precisely where the bomb went off a few minutes later, says the BBC's Jonathan Head in Bangkok.
"There is a suspect… we are looking for this guy," Prime Minister Prayuth Chan-ocha told reporters.
Authorities were "quite close" to identifying the suspect, Thai government spokesman Maj Gen Weerachon Sukhontapatipak told the BBC, but other leads were also being pursued.