Police admitted Wednesday the blast near Sanam Luang which injured two women on Monday was caused by a low-impact bomb on a timer, saying their initial claim that explosive materials were not used was a tactic to keep the investigation under wraps.
Pol Lt Gen Sanit Mahathavorn, chief of the city's police force, said investigators wanted to conduct the investigation in secret but had now withdrawn their earlier claims that the PVC pipe had spontaneously exploded.
A thorough inspection of shrapnel and other materials at the crime scene has confirmed the blast resulted from a bomb which was small enough to stir up unrest but not likely to cause any casualties, deputy national police chief Srivara Ransibrahmanakul said.
Police earlier dismissed rumours the blast was a bomb plot because no traces of soot were found.
Pol Gen Srivara said Wednesday no soot was detected but this may have been because of the rain and the small size of the explosive material used. He said police now suspect the bomb was placed inside a polyvinyl chloride (PVC) pipe.
Though the latest examination indicates the device was only as powerful as some firecrackers, as Pol Gen Srivara described it, investigators have vowed to hunt down the perpetrators.
The bomb went off on Monday night outside the National Theatre near Sanam Luang, causing slight injuries to two passersby.
The timing of the blast led officers to question whether it was part of a larger effort to try and discredit the military-sponsored government as it came ahead of the upcoming third anniversary of the 2014 coup.
Pol Gen Srivara said the bomb makers' chief goal was probably intended to fan public disorder but that it was too soon to say with certainty that the blast was politically motivated.
Police investigators are using remnants of the explosion and footage from security cameras installed near the crime scene to help them track down suspects.
Experts found the device had an integrated circuit (IC) timer and bore traces of potassium chlorate, a popular chemical used in making bombs.
Officers initially believed it was similar to the low-impact pipe bomb that exploded in front of the old Government Lottery Office and wounded two women on Ratchadamnoen Klang Avenue on April 5.
It also bears similarities to bombs used during the pro-Thaksin red-shirt rally in 2010, and insurgent attacks in the South in 2007 and 2008, Pol Gen Srivara said.
Officers will next inspect the wiring and see if the bomb's construction links it to previous blasts or known insurgents, he said.
"Things will become clearer in the next three days," Pol Lt Gen Sanit said.
Chana Songkhlam police are examining security footage for more clues, he said.