Four suspects already arrested in the case have given police valuable evidence and have implicated the individual behind the incident in which Kobkul died in a hail of automatic gunfire as she was being driven home.

The chief investigator in the case, Police General Phreophan Damaphong, said police were working hard to produce evidence to implicate the so-far-unidentified individual.

"Everyone knows now who he is, so do I and so do the reporters," he said, without offering a hint of the man's identity.

Anont Phanrat, the first suspect arrested on Wednesday, volunteered information that led to the arrest of three others. He has been offered immunity from prosecution in return for his testimony against the three others and the mastermind, and is now under heavy police protection.

Acting on fresh details provided by Anont, Phreophan said police plan to issue a warrant for the arrest of a man referred to merely as "Love". According to Anont, "Love" was actually driving the pickup carrying the two alleged gunmen, Jamron Omthong, a Phetchaburi kamnan, and his son Suppharit, when the murder was committed.

Anont originally told police he was the driver of the pickup. Police later found a black Toyota Vigo parked near the Thai-Burmese border in Suan Phueng district, and are investigating whether it was the one used in the murder and whether "Love" was driving it, as Anont now claims.

Police Maj-General Assawin Khwanmueng, a deputy commander of the Central Investigation Bureau who is assisting with the case, said Anont's conflicting information will not affect the investigation or the prosecution later.

"He is merely trying to distance himself from the case in expectation of receiving a lesser court penalty," he added.

Police are now searching for Phongphat Narknoo, a member of the Don Tako Tambon Administrative Organisation and the owner of a house in which Sa-ngad Phumpheng, said to be one of the organisers of Kobkul's murder, drank poisoned beer. He was later pronounced dead at a local hospital. Phongphat is sought for questioning over Sa-ngad's death.

A spokesman for the Police General Hospital, Maj-General Somyos Deemark, said the results of a forensic examination, which will be known within the next few days, will reveal whether Sa-ngad voluntarily drank the poisoned beer or it was forced into his mouth.

Police have sent an image of a suicide note, written in charcoal on a plywood wall in the house where Sa-ngad was found unconscious, to Chulalongkorn University for linguistic and psychological experts to determine whether it was written by Sa-ngad, written voluntarily or written under emotional stress.

Sa-ngad's brother and son are also cooperating with police to identify the man's handwriting. His son Narong said he did not believe Sa-ngad had killed himself and also dismissed suggestions that his mother, Thassawan, who last saw Sa-ngad drinking alone, had hanged herself out of distress or fear for her life.

Meanwhile, police who searched a small patch of forest near a factory in Muang Ratchaburi district found a number of spent M-16 cartridges with serial numbers that matched those found at the crime scene. The area was reportedly used as a firing range by the suspects a few days before Kobkul's murder.

A senior police forensic expert, Maj-General Prawit Sinlapasu-wan, said the spent cartridges found at the crime scene did not match those from similar assassinations and violent crimes in western provinces.

Police divers earlier found two M-16 magazines in the Mae Klong River, one empty and the other fully loaded with 20 rounds. The magazines and bullets are new and could have been used in the murder.

A new automatic pistol, well wrapped in plastic, was also found in the river and is undergoing scientific testing and registration verification to identify its owner.