A M-67 grenade was found in a bag at the Attorney General's Office on Friday in the latest string of bomb attack and threats which took place in recent days at several government offices.
Bomb squads successfully defused the explosive. Police are now checking the security cameras in their attempt to find suspects.
Meanwhile Army spokesman Col Sansern Kaewkamnerd said police are assigned to strengthen security measures on 68 expressways across the capital.
The moves were made after several bomb attacks at the government agencies were believed to be fired from expressways.
The spokesman said the security authorities will coordinate with the Expressway Authority of Thailand to examine its 180 closed circuit televisions, as well as 26 CCTVs on the Don Mueang Tollway to use in the investigation.
Nearly 70 checkpoints will be set up on expressways after unknown attackers used expressways as locations to fire grenades at key government offices to discredit the government, Col Sansern said.
The attack is unlikely to target any individuals, so the public should not be alarmed, he said.
According to CAPO, 12 explosions occurred in Bangkok, Nonthaburi and Samut Prakan and four others in the northern province of Chiang Mai after the Internal Security Act (ISA) was put in place on March 11 and extended until March 30.
