Despite new surveillance video that may offer a possible clue to the bombing in central Bangkok that killed 20 people, Thai police said Sunday that the perpetrators may have already fled the country, and that they will "need some luck" to catch them.

Surveillance video leaked to Thai media shows a man in a blue shirt placing a bag on a riverside walkway, then kicking it into the water on Monday night shortly after the explosion several kilometers (miles) away at the downtown Erawan shrine. About 18 hours later, at 1 p.m. Tuesday, an explosion took place at the same spot near a busy pier, causing no casualties.

Col. Winthai Suvaree, a spokesman for Thailand's ruling junta, also said that closed circuit television showing the main bombing suspect was used to trace the route he took to and from the site of Monday evening's rush-hour attack. He said that a police sketch of the suspect had been distributed to border posts.

Police spokesman Prawut Thawornsiri said he believes the perpetrator would have timed an escape carefully and "wouldn't have much time to stay around."

"I suspect that he may have left, but we will keep searching, in case we can find others who may be in the country or find clues, evidence and witnesses who may have seen him," he told Channel 3 TV network.

Police have offered a reward that on Friday was raised to 3 million baht ($85,000). On a police arrest warrant, the suspect is described as a "foreign man," although a military spokesman said a connection to international terrorism seemed unlikely.

Thailand's police chief Somyot Poompanmoung said investigators would need some luck to catch those behind the attack.

"I have to say we need some luck. If the police have good fortune, we might be able to make an arrest, but ... if the perpetrator has good fortune maybe they can get away," he told reporters Sunday after a ceremonial show of security strength meant in part to reassure the public over safety.

Meanwhile, authorities have arrested two people for allegedly spreading "false information," apparently in connection with the shrine bombing.

A student who posted a bomb threat on Facebook was arrested in Ayutthaya on Saturday and was charged with a computer crime that is punishable by up to five years in jail and a fine of 100,000 baht ($2,800), Winthai said. He did not give details of the second arrest except to say it happened in Bangkok. Ayutthaya is about 80 kilometers (50 miles) north of Bangkok.

Winthai said the two people were arrested for spreading "false information causing confusion in society," but did not say whether their posts were directly linked to the bombing.

So far the operation to find who carried out the attack appears to have made little headway, with apparently contradictory statements coming from the military-backed government and the police.

Theories abound as to who was responsible for the attack.

They include ethnic Uighurs (pronounced WEE-gurs) angry that Thailand repatriated to China more than 100 of their countrymen who had fled from there; Islamic separatists who have been carrying out an insurgency in southern Thailand for a decade; frustrated supporters of former Prime Minister Thaksin Shinawatra angry at the military government that opposes his return to politics; and rival factions within the army contending for power.