One sergeant-major acted as the identifier of the target for Lieutenant Thawatchai Klinchana, who was driving a Daewoo sedan loaded with explosives on August 24, while the other was identified only as Sgt-Major Chor, who is still at large, a source said.

The two officers were identified as Colonel Sor and Lt-Colonel Mor, the sources said. Some or all of the four were near the flyover at the Bang Phlat Intersection when uniformed officers intercepted the Daewoo.

Two other sergeants-major are suspected of devising the circuitry used with the car bomb, but there is no evidence directly implicating them in the assassination plot.

Since Thawatchai vowed not to name anyone behind the conspiracy, police are relying more on circumstantial evidence, especially a pickup belonging to Sgt-Major Chor seized at the man's home.

A source said the police's plan to conduct simultaneous raids on several locations to look for evidence had been put on hold, after the key investigators in the case reached an agreement that evidence needed to be confirmed before the investigations were conducted.

An Army ordnance expert, who asked to not be identified, questioned the police estimate of one kilometre as the lethal range of the car bomb.

He said the blast at an Army arms depot in the Saphan Daeng area in Bangkok in 1980 was far less dangerous, even though 1.5 tonnes of C-4 composite B explosives and a huge quantity of rockets were stored there. But residential communities just a few hundreds metres away suffered little damage.

Thawatchai is increasingly unwilling to cooperate with police after he said he would take sole responsibility for driving the bomb-laden car around. He does not talk with police without at least one of his three attorneys present.