Pol Lt Col Prateep Suksarn, a duty officer from Bacho police station, said the explosion occurred at 9.50am after the bomb disposal team were called to investigate reports of explosives hidden in a hole on Phetkasem Highway.
Members of the provincial bomb squad had rushed to the scene, near Ban Sompoy, tambon Kayo Mati in Narathiwat's Bacho district, following a report that a bomb had been planted at the bottom of a hole on the side of the road.
The bomb squad turned on a mobile phone signal jammer while examining the hole, and confirmed that it did contain explosives.
But moments later a bomb inside a 50-kilogramme gas cylinder buried nearby exploded, hurling the bodies of Pol Sub Lt Chaen Warongpaisith, deputy leader of the squad, Pol Sub Lt Jaroon Mekruang and Pol Sgt Nimit Deewong about 40 metres from where they had been working.
A chief warrant officer identified as Somkiat Youngsiri of Narathiwat Task Force 32 was also wounded in the powerful blast, which damaged a Toyota pick-up truck containing the signal jammer used by the bomb squad.
After the explosion, Pol Sub Lt Polawat Thepsorn and other bomb disposal team members continued to scour the scene and found another bomb inside a 50kg gas cylinder, connected to wires leading to nearby trees. They destroyed it with a water cannon.
Narathiwat police chie Pol Maj Gen Pattanawut Angkhanawin said car tyres had been set fire to on the highway on Sunday night to lure authorities into examining the scene the next morning.
Pol Sub Lt Chaen dies in the line of duty
Pol Sub Lt Chaen, a 50-year-old resident of Narathiwat, was regarded by many people as a hero because of his 'bravery and commitment to his duties'.
He had defused at least 200 explosives in the restive South, and was always one of the first in line to do the job, according to Isra News.
He received several awards for his work, such as the Outstanding Police Officer Award from the Royal Thai Police in 2009 and the Good People of the Country Award from the Khon-Kon-Khon (Man Discovers Man) television programme in 2011.
"I'm committed to my work because I hope to see peace in the three southern border provinces, where I was born," Pol Sub Lt Chaen told Isra News Centre earlier.
"I want everyone to live peacefully and happily as before. My work philosophy is that I'm ready to dedicate myself for my brothers and sisters so they can sleep well."
After receiving the Good People of the Country Award in 2011, he and six of his colleagues were injured in a bombing in the far South two days later. Later that year, he was again injured in a gun attack believed to be connected to the southern insurgency.
Isra News Centre said the bomb disposal specialist survived three gun attacks and five bombings.
Pol Sub Lt Chaen graduated from the Provincial Police Training 9 School in 1985, and passed bomb detection and disposal training in 1989. A year later, he was seriously wounded in a shooting in Yala. He was subsequently transferred to work in Narathiwat.
He was relocated to Ayutthaya in 1999, but soon after asked his superiors to return to work in his home province. His transfer back was approved.
He was promoted to the rank of sub lieutenant on June 8 this year.
Teacher's abductor caught
Meanwhile in Pattani's Yarang district, a combined force of police, rangers and administrative officials arrested a suspect in the abduction of a teacher in 2006.
Government forces raided a house in Darusalam village of tambon Mo Wawi in Yarang district about 2am on Monday based on information that Mayiding Jaru, 29, a suspect wanted on a court warrant for the shooting of Pol Sr Sgt Maj Phurit Wanthong in the village on Feb 12, 2007 was hiding there.
The security team did not find Mayiding but instead arrested Ahama Kajeh, a native of Narathiwat's Rangae district, who was wanted on two warrants.
Under the first warrant, Ahama was suspected of involvement in the May 2006 abduction of Juling Pongkanmoon, 24, a teacher at Kuchingrupa School. She was later found beaten to death.
The other warrant related to a shooting on a train on June 21, 2008, in which three railway officials and a police officer were killed and a policeman injured.