CCTV video widely circulated by local media Saturday showed a man in a blue t-shirt kicking the package off a footbridge in the same spot where a device exploded on Tuesday without injuries.
That second blast intensified anxiety in an already rattled city, following Monday's bomb attack on a religious shrine in Bangkok's commercial heart that cut down 20 people and wounded scores more.
Authorities have not ruled out a link between the two explosions.
The shrine bomb killed mostly ethnic Chinese tourists from across Asia, and has left police scrambling to find the assailants.
National police spokesman Prawut Thavornsiri confirmed that the new footage was being examined to decide whether the man dropping the package into the water near Saphan Thaksin pier was a suspect.
"We have to verify the information," he told told AFP adding the situation was "still confusing".
The unverified footage is time-stamped just a few minutes after Monday's blast, which struck at 18.55pm (1155 GMT).
- Bag kicked into canal -
It shows the man in the blue t-shirt carrying something heavy in a plastic bag. He goes to the side of a footbridge, places the bag down and then uses his mobile phone.
Around a minute later he pushes the bag into the canal with his foot, kicking up a visible splash of water.
The following day, shortly after 1:00 pm on Tuesday, an explosion went off in the canal, which is near a popular tourist pier, sending people scurrying for cover but causing no injuries.
Until now the search for the perpetrator of Monday's attack has focused on a primary suspect in a yellow t-shirt seen placing a heavy rucksack under a bench at the shrine.
Moments later the blast struck, scattering body parts, glass and debris over a bustling Bangkok shopping district.
Asked whether the man in the blue t-shirt and the yellow t-shirt could be the same perpetrator, Prawut said: "It's likely the two men are different."
A junta spokesman said 56 people remained in hospital on Saturday. Many are known to have suffered horrific injuries.
- Wailing mourners -
At a Catholic church shaped like a Thai temple less than 10 minutes from the blast site, around two dozen worshippers attended a memorial service for Di Wu Chengi, a Christian Chinese mainlander who was killed in the blast.
Wailing could be heard coming from inside the church as the congregation sang the hymn "Nearer my God to thee".
"We are all here because we feel sorry not only for Di Wu Chengi but also for society experiencing violence," a priest told members of the congregation according to an AFP reporter at the scene.
"The loss of his life is a witness to the fact that violence is not an answer," he added.
With no-one claiming responsibility for the bombing, rumours and speculation have swirled in Thailand over the country's worst single mass casualty attack in living memory.
The potential perpetrators named by police and experts alike include international jihadists, members of Thailand's southern Malay-Muslim insurgency, militants on both sides of Thailand's festering political divide and even someone with a personal grudge.
Bangkok is in the grip of a decade-long political crisis that has been punctuated by violence, albeit mostly shootouts and small grenade or pipe bomb attacks.
On one side stands the military, backed by the middle class and elite.
On the other, the rural and urban poor loyal to populist politician Thaksin Shinawatra, toppled in a 2006 coup, and his sister Yingluck who was forced out of office days before the current junta seized power last year.
Thaksin's eldest son said he had more than doubled the reward offered by police for the capture of the main suspect by putting up nearly $200,000.
The total reward money on offer from both police and private citizens now stands at around $335 000.
© 1994-2015 Agence France-Presse
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