Investigators have already been assigned to check security cameras at the hospital parking building in Pathumwan where Dr Tul left his car and in the surrounding area to find the culprits, Pol Lt Col Sorakarn Damkrabi, a police chief of Pathumwan police station, said.
Meanwhile Pol Capt Kaneungnuch Thotsapairin, who is overseeing the case, said police would summon witnesses for questioning.
Pol Col Kamthon Uicharoen, a bomb expert, said the circuit was incorrectly connected. The device was a grenade thus a circuit was not necessary. However he said the explosive was intact. He said the culprits might have simply intended it as a threat.
Dr Tul filed a complaint with police on Monday after he found the explosive while he was about to leave the hospital to pick up his children at school.
He told police that when he looked inside the bag, he found a round tube with an explosive-like object and circuits. However, he thought the circuit was not complete and was fake, so he threw it away.
He decided to file a complaint with police after having it checked and finding it intact. Tul is the leader of the multi-coloured movement that has campaigned against former ex-Prime Minister Thaksin Shinawatra and the red shirts movement.