The police have now believe the explosion in front of the National Stadium on Monday night was caused by a pipe bomb after bomb scene investigators found an integrated circuit (IC) timer near the scene.
The IC timer was collected at a spot 15 metres from the explosion scene.
Police investigators said the device was as same as that was used by perpetrators in the pipe bomb explosion in front of the old Government Lottery Office on the night of April 5 on Ratchadamneon road.
The evidence indicated the two explosions believed to be the work of the same group.
But security officials said the pipe bomb explosion at the National Theatre was not intended on lives but just to create disturbance same as the explosion at the old lottery office.
Police investigators are now scouring all CCTV footages at the scene and adjacent areas to find the perpetrators.
Source: Thai Public Broadcasting Service (Thai PBS)