The killing of a man convicted of murdering five members of a family in Songkhla 18 years ago was not an act of vengeance, police said.
Netirat Noppawong, alias Sak Pakro, was shot in front of his house in tambon Pakro of Songkhla's Singha Nakhon district on Sunday.
He succumbed to his injuries on the way to Sadao Hospital.
At least two perpetrators were involved in the attack on Netirat, according to investigators.
The attackers stopped their car in front of Netirat's house and shot the victim twice before fleeing.
Netirat was convicted of colluding with Songkran Kaew-ubon to rob and murder the family of Prapas Boonthawee, chief of a health centre in tambon Rawa of Ranot district, on April 25, 1997.
The five bodies were found hanged from a staircase.
Revenge for the family's killing 18 years ago has been ruled out as a motive for Netirat's murder, said Pol Col Kittichai Sangkathavorn, chief of Sadao police office.
Police were focusing on his conflicts with illegal businesses in the border district of Sadao, he said.
Netirat was involved with several groups of influential people there, he said.
According to Pol Col Kittichai, Netirat was also once arrested for allegedly encroaching on a forest reserve in Sadao.
Police are tracking down the vehicle used in the attack, but more CCTV footage needs to be reviewed, he said.
The gunman was believed to be a hitman as he shot twice from the vehicle and both rounds hit the victim, Pol Col Kittichai said.
After murdering Prapas's family, Netirat was apprehended in Kanchanaburi's Muang district in May 1997.
He admitted to the killings.
The Criminal and Appeal courts handed down the death penalty, but the Supreme Court sentenced him to life in prison.
Netirat's jail term was gradually reduced because of his good behaviour behind bars and he was eventually released two years ago.