The officer did not identify the three officers or the names of units they had been assigned to.

Winai said police were still treating Sgt Major Chakhrit Janthara as a suspect, not as a prospective witness, who might be granted immunity from prosecution in exchange for his cooperation and information against the other five suspects.

The CSD chief said police were compiling scientifically-based evidence against the five suspects and would wrap up the case in the next few weeks.

Pol Maj General Assawin Khwanmueng, a senior investigator handling the case, theorised that the remote-controlled detonation did not work because the person who activated the trigger device was too far from the Daewoo sedan loaded with explosives, or the person feared the impact of the explosion because he was too near to the car. He did not say what the actual test results were.

Assawin was responding to news reports citing military sources that the remote-controlled unit could never work because the electronic circuit attached to the explosives cache in the Daewoo sedan was switched off.

Phaholyothin police, meanwhile, said police had not issued a warrant for Maj General Phairoj Theerapharb, one of the five suspects released on bail, for not cooperating when police wanted to take his fingerprints the day he surrendered earlier this month.