Shortly before police were to apprehend him yesterday, a key suspect in the murder of MP Kobkul Nopamornbodi in Ratchaburi on Saturday drank poison-laced beer and died, leaving a suicide note acquitting others who had already been arrested.

Four suspects have been apprehended. One is Jamron Omthong, a kamnan in Ban Lad district in neighbouring Phetchaburi province. Another is his son, Suppharit.

The alleged driver of a pickup truck carrying the two gunmen who blasted Kobkul's car was Anont Phanrat. The fourth suspect is Anantasak Srisawas, a member of the Muang Ratchaburi municipality.

The man who died after drinking beer laced with pesticide is Sa-ngad Phumpheng, who left a suicide note written with a pencil saying he was solely responsible for the murder. Police are investigating whether Sa-ngad wrote the letter, had it dictated to him or whether it was faked.

The letter read: "Everything was carried out all by myself. Anantasak and the others had nothing to do with it. I had helped MP Kobkul all along but she had betrayed me, so I got angry. The senator, I apologise for doing things without letting you know beforehand. I did it alone. Please take care of my kids, my wife and my cows. Really sorry."

Jamron, 50, was the first suspect arrested after police apprehended him at his home in Phetchaburi at 5am yesterday morning and found two shotguns, two pistols and ammunition.

The details of the assassination were provided mostly by Anantasak, the manager of Sri Sawat Market. Quoting Anantasak, 42, police said Jamron sat in the passenger seat of the pickup used in the murder, while Suppharit and Anantasak hid in the flatbed among baskets of vegetables. Sa-ngad, who was to identify Kobkul, was driving another vehicle.

The shooting occurred at an intersection when Kobkul's vehicle stopped for a red light. Anont pulled up alongside Kobkul's pickup. Jamron, carrying an M-16 with folded stock, and his 26-year-old son, with a standard M-16, jumped out and sprayed bullets into the passenger cabin. Neither murder weapon has been found.

Sa-ngad's wife Thassawan told police he was drinking beer alone at the house of a local politician in the Don Tako tambon of Muang Ratchaburi district when she last saw him. Sa-ngad was later found lying unconscious with saliva coming out of his mouth. He was pronounced dead shortly after being admitted to Suan Phueng Hospital. An autopsy is under way to determine whether Sa-ngad killed himself or was forced to drink the pesticide.

A warrant is being sought to summon Phongphat Narknook, a member of Don Tako tambon administrative organisation and the owner of the house in which Sa-ngad was drinking, to question him over his relationship with Sa-ngad.

Police spokesman General Ajiravid Subarnbhesaj said investigations are centring on whether the two gunmen belonged to any guns-for-hire groups.

He said police had sufficient evidence against the four men arrested. "They are not scapegoats," he said.