By Jerome Taylor and Anusak Konglang
Thai police are investigating new CCTV footage showing a man dropping a package into a canal as the hunt continues for those behind Monday's deadly Bangkok bomb blast.
The security video widely circulated by local media on Saturday shows 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.
Police are convinced the attack was planned and co-ordinated by a network and insist their investigation is making progress despite days of unclear and sometimes conflicting statements.
National police spokesman Prawut Thavornsiri confirmed the new footage was being examined to decide whether the man dropping the package into the water near Saphan Taksin pier was a suspect.
"We have to verify the information," he 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 6:55pm (11:55pm NZT).
It shows the man in the blue T-shirt carrying something heavy in a plastic bag. He goes to the side of a footbridge, puts the bag down and then uses his mobile phone.
About 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 1pm 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.
The search for the perpetrator of Monday's attack has focused to date on a primary suspect in a yellow T-shirt seen placing a heavy backpack under a bench at the shrine.
Moments later the blast struck, scattering body parts, glass and debris over a bustling Bangkok shopping district.
Prawut said on Saturday there was no further update on the investigation into the main suspect, who has been described in an arrest warrant as an unnamed foreign man.
Asked whether the men in the blue T-shirt and the yellow T-shirt could be the same person, he said: "It's likely the two men are different."
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.
Uighur militants from China have also been mooted as possible perpetrators as the shrine is popular with Chinese visitors.
Late Friday, former Prime Minister Thaksin Shinawatra'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 US$200,000.
The total reward money on offer from police and private citizens now stands at about US$335,000.