Police have gathered more evidence against three people suspected of setting fire to a portrait of His Majesty the King and damaging state property outside Klongprem Central Prison in Chatuchak district in the early hours of Sunday, according to the Ministry of Justice.
The trio, two men and one woman, is to be charged with arson, trespassing on state property and violating Section 112, or lese majeste, according to Thanakrit Jitareerat, a close aide to Justice Minister Somsak Thepsutin.
The crimes were reported about 3.10am on Sunday.
The suspects were reportedly recorded by surveillance cameras and found at the scene with a lighter and some fuel.
The Corrections Department has lodged a complaint with Pracha Chuen police station against the three people captured on camera.
Police are now gathering more evidence to support their request for arrest warrants against the suspects.
The investigation is expected to be expanded as a political group is suspected to be behind the arson. according to police.
Ayuth Sintoppant, director-general of the Department of Corrections, said police identified the vehicle used by the three suspects and tracked it to where they live.
Mr Ayuth said that the vehicle was the only car that passed through the area at about 3.10am on Sunday.
He has reportedly set up a fact-finding committee chaired by the department's inspector-general and asked security forces to beef up security outside of prisons.
Mr Somsak has told him to tighten security and never let a similar incident happen again.