The explosion which occurred in front of the Unesco office Friday morning was caused by a ping pong bomb, Thong Lor police chief Pol Col Kajonpong Jitpakpoom said.
A street sweeper of the City Hall sustained minor injuries from the explosion and was treated at the scene.
The city street sweeper stumbled a black plastic bag while sweeping the Sukhumvit road in front of Soi 40. She opened the bag and found a round object wrapped in black plastic tape.
She kept the bag to herself but, somehow, it fell on the road surface and exploded, slightly injuring her.
Pol Col Kajonpong said the ping pong bomb might be dumped on the road by one of the students out of fear that he might be caught by police for illegal possessing of an explosive device.
The forensic police also found many knives hidden in the bush where the explosion took place.
Source: Thai Public Broadcasting Service (Thai PBS)