At least 2,500 deaths occurred during the three-month campaign in early 2003 and Sunai Manomai-udom from the Department of Special Investigation (DSI) is looking into possible links between the ex-PM's speeches to murderous police actions.

Sunai said he wanted to work with the Office of Narcotics Control Board (ONCB) and newspapers editors to review Thaksin's speeches. He is trying to prove that Thaksin gave the police the go-ahead to implement "iron fist" operations.

The DSI is analysing Thaksin's statement in which he said: "During the three-month suppressions, if drug dealers are going to die, it's normal. Even after they are killed their assets should be seized; we have to be equally brutal against them.

"Drug dealers have two places to go to: jail or the temple morgue. You must use an iron fist and merciless decisiveness against those who deal drugs. Narcotics are a major threat to the national security and we need to declare war on them to achieve a complete victory. It's necessary to see some casualties."

A Justice Ministry source said if a legal study found Thaksin encouraged killings through his speeches, the populist leader could face criminal charges for encouraging, glorifying or promoting murder.

Some figures claim that 1,161 killings were carried out by the police and the remaining were "internal drug trade murders", where dealers tried to silence each other.

Sunai said the DSI had clear evidence that police carried out four murders and officers involved would be asked to account for those deaths.

Officers involved in the death of a young boy, who was killed by a stray police bullet during a drugs bust in Bangkok, would also face prosecution - for alleged doctoring of evidence, perjury and abuse of power, he said.