The fatal shooting of two security officers in Pattani's Mae Laen district was allegedly carried out by a fellow ranger, an Internal Security Operations Command (Isoc) Region 4 spokesman says.
The incident took place on Tuesday as six rangers conducted a night patrol in Ban Kao Sai in tambon Mae Laen.
Col Pramote Prom-in said initial reports suggested the group was ambushed by insurgents, but it was found later the shooter might have been volunteer ranger Sarawut Kampan, 21.
The slain rangers were identified as Pvt 1st Class Ronnawit Singsai, 30, and volunteer Kriangsak Thupphutsa, 29.
The suspect had been arrested and was being questioned by police, Col Pramote said.
Col Pramote said stress among security officers in the South is common and the army chief has ordered superiors in every military unit to organise psychological check-ups for officers every month.
Police yesterday took ranger Sarawut to perform a crime re-enactment at the scene.
A probe into the killings found Mr Sarawut was infuriated when he was reprimanded by his superiors during a patrol on their motorcycles.
Mr Sarawut parked his motorcycle under a bridge and allegedly gunned down Kriangsak first with his M16 rifle and then shot and injured Pvt 1st Class Ronnawit who was standing on the road.
Other rangers fled and took cover as they believed they were being ambushed by insurgents, police said.
Mr Sarawut then allegedly approached Pvt 1st Class Ronnawit and shot him dead at point-blank range, police said.
Mr Sarawut then asked police officer Pol Snr Sgt Maj Sukjai Chusinui to take him away from the scene on his motorcycle.
Mr Sarawut allegedly pointed the rifle at Pol Snr Sgt Maj Sukjai's back during the ride. While passing Mae Laen police station, the policeman managed to shove Mr Sarawut off his bike and called for his colleagues to help arrest him.
He is now being detained in a cell at Mae Laen police station.
Meanwhile, a military base of the Narathiwat 30 Special Task Force in tambon Sawo of Narathiwat's Rueso district was attacked with two grenades fired with an M79 launcher about 10.30pm on Tuesday.
No military officers were injured in the attack.
Meanwhile, Kalapo School in tambon Trobon of Pattani's Sai Buri district reopened yesterday after its closure following a bomb blast which killed two rangers at a makeshift shelter next to the school on Monday.
The school's headmaster Songkran Jariyapianpan said only 50 students turned up so he decided to cancel the classes.
The school will reopen on Monday, he said.
Barisan Revolusi Nasional spokesman Abdul Kareem yesterday posted another YouTube video calling on the military to stop hurting Muslim leaders, Muslim teachers, imams and local leaders.