Police believe technical school students left the ping-pong bombs in a Sukhumvit area that blew off the finger of a City Hall rubbish collector.
Police insist the explosion had nothing to do with any political conflicts.
Three rubbish collectors were injured in the explosion that took place on Soi 4 off Sukhumvit 101/1 in Bangkok Tuesday morning when the workers were loading rubbish in front of a house into their truck.
The victims were identified as Surat Sitthipanya, 21, who had the forefinger from his right hand blown off, Amporn Sae Bay, 47, and Koon Jiangchai, 44.
Mr Amporn and Mr Koon sustained minor injuries to their legs by the shrapnel of the bombs.
All were admitted to Bang Na hospital.
Police, the bomb squad and rescue teams were sent to the scene, who identified the bombs as homemade ping-pong bombs.
Three more homemade ping-pong devices were also found at the scene.
The bombs were defused and collected for further investigation.
National police chief Somyot Pumpanmuang said he believed the blast had nothing to do with political conflicts.
The bombs were probably discarded at the scene by youngsters who were scared in case police found them in their possession, he said.
Police searched a house close to the scene after receiving a tip-off that groups of youngsters tend to congregate there. Officers found eight swords and a pistol in the house but no trace of explosive items.
Police are now examining security camera footage in the area to find the suspects, he said.
The victims told police that while they were collecting rubbish from house number 120/542, they noticed 4-5 suspicious round objects bound with black tape in a green plastic bag.
Mr Surat tried to remove the tape to separate the rubbish when the bombs went off.
Pol Maj Gen Nipon Charoenpol, deputy chief of the Metropolitan Police Bureau, said ping-pong bombs are often used by technical school students.