Bangkok: Thai authorities have gathered evidence linking Monday's shrine bombing to another blast in what appeared to be a co-ordinated attack on the Thai capital.
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Less than 30 minutes after a blast tore through tourists at the Erawan shrine, a man kicked a package into a canal at a pier near the Chao Phraya river several kilometres away.
CCTV footage has emerged showing the man wearing a blue T-shirt and fiddling with a mobile telephone.
A bomb packed with high explosives and ball bearings - similar to the one at the shrine - exploded in the canal the next afternoon. No one was injured in that blast, although the area is popular with tourists.
After much criticism of the Thai authorities over their conflicting and confusing statements after the shrine bombing, officials have become more cautious in their public statements.
Police spokesman Prawut Thavornsiri said it was too early to conclusively link the man at the Sathorn-Taksin pier with the shrine bomber.
"The thing he kicked in the canal may be just garbage. We don't know yet," he said.
But Thai media quote sources saying police believe the second bomb was intended as a back-up in case the Erawan bombing was foiled or to create fear by showing the perpetrators were capable of a rapid follow-up attack.
Separate security footage now points to three men who are wanted for questioning over the bombings.
The key suspect is a yellow-shirted man, possibility wearing a wig, who mingled casually with tourists at the shrine, slipped a backpack from his shoulders and left the scene minutes before the blast that left 20 people dead and more than 120 injured.
The second man shown in footage at the canal appears to be aged 30 to 40 and about 170 centimetres tall.
Security footage also shows a third man suspiciously taking photographs at the canal moments before the second bomb exploded there.
Police believe the canal bomb was concealed to resist water and an electric circuit could have delayed detonation.
Thailand's military government claimed soon after the shrine bombing that it was "unlikely" that international terrorists were involved.
But Thai media now report officials saying foreign techniques were used in the both the shrine and Sathorn-Taksin bombs.
"Such a technique did not exist in Thailand before," a source was quoted as saying. TNT and ball bearings were packed in a pipe in both bombs.
Thailand's military government has ramped up a public relations campaign in an attempt to head off mass cancellations of tourist bookings in response to the bombings, particularly from China.
Ethnic Chinese suffered the most casualties in the shrine bombing, the largest attack in Thailand in years. Up to 7 million Chinese tourists were expected to visit Thailand in 2015.
Government spokesmen have been making reassuring announcements in Chinese as well as Thai.
Senior police have also been making walkabouts in areas popular with tourists, including Bangkok's seedy red light districts.
Tourism accounts for 10 per cent of Thailand's economy, which had already suffered a sharp downturn.
Theories on who was responsible for the attack abound but investigators appear to have few leads.
They include ethnic Uighurs angry that Thailand deported 109 of their countrymen to China; returning jihadists from the Middle East; ethnic Malay separatists waging a bloody insurgency in southern Thailand; disgruntled factions in the police or military; and opponents of the military junta that seized power last year.
Fifty-six of the bombing victims remain in hospital.
