Pol General Srivara Ransibrahman-akul said police were thoroughly investigating the evidence, including security camera footage at the scene.

He said the probe should be finished in three days. He is the first state official to confirm that the loud noise heard on Monday night was actually from an explosion.

Prime Minister Prayut Chan-o-cha, meanwhile, said he would wait for forensic opinion before offering a comment on the incident.

If the incident was a bomb attack, the government will go after the culprits, he said, adding the public should help the authorities if they spotted something unusual.

Srivara said the bomb was composed of potassium chlorate with a small quantity of dynamite hence no traces could be found at the scene.

“The bombers had knowledge of using an IC [integrated circuit] timer in assembling the bombs. The nature of the attack and the material used |are similar to the April 5 explosion in front of the old Lottery Government Office buildings,” he said. He said he could not yet confirm if the same groups were behind both attacks.

Meanwhile, investigators yesterday checked the scene again and found electrical wires and coin cell batteries, which could be remnants from the integrated circuit timer used in the explosive.

The evidence will be compared with those collected from the April 5 explosion.

City police chief Pol Lt-General Sanit Mahatavorn said yesterday that his initial response, describing the explosion as being caused by a falling signboard, was aimed at misleading the culprits.

He said the strategy on Monday was to allow police to conduct a secret investigation.

Sanit went to the scene after a blast was reported in front of the National Theatre on Rajdamnoen Road. Although two people were injured from the presumed blast and an advertisement sign was damaged, Sanit maintained at the time that the loud noise was caused by the falling and breaking of an advertisement sign.

He said on Monday that bomb disposal officials had checked the site and did not find any traces of explosives.

Yesterday he said that statement was aimed at misleading the bombers into believing that police would not pursue the investigation.

“Actually, police have secretly investigated the incident and components of explosives were found at the scene that night,” he said.