Kowit's signature, which is the final step in indicting the military officers, came after an hour-long meeting with several senior police officers, including chief investigator of the case, Central Investigation Bureau head Lt General Montree Jamroon.

The four suspects are Major General Phairoj Theerapharb, Colonel Suraphol Supradit, Lt Colonel Manas Sukprasert and Lieutenant Thawatchai Klin-chana, all of whom are attached to the Internal Security Operations Command (ISOC).

The officers all face six charges: criminal conspiracy to commit premeditated murder; attempting to kill a state official on duty; possessing explosives; having non-authorised items in their possession; forging and using official documents; and criminal conspiracy.

Sgt-Major Chakhrit Janthara, of ISOC, whom police say was responsible for obtaining a Daewoo sedan allegedly found carrying a large amount of explosives and a blasting agent, would not be charged in exchange for turning to give evidence for the state against the four officers.

Phairoj, the only suspect not in military custody, is required to report to a military prosecutor soon.

The investigation report for the case is over 3,000 pages long and includes interviews with 106 witnesses.