Gunman identified as Sergeant Major Jakapanth Thomma stole army vehicle and posted photos of himself in tactical gear.
A Thai soldier has killed several people in a shooting rampage in the northeastern city of Nakhon Ratchasima, according to police.
Police identified the suspect of Saturday’s attack as Sergeant Major Jakrapanth Thomma and said he remained at large.
Local media said at least 12 people were killed, while the AFP news agency quoted health authorities as saying that the death toll stood at 17.
The suspect reportedly stole an army vehicle and posted photos and video of himself in full tactical gear as the attack was carried out.
He wrote on his Facebook page earlier in the day that “Death is inevitable for everyone.” The suspect also posted a picture of what appeared to be his hand holding a gun.
At one point after the shooting had begun, he wrote online, “Should I give up?” before his account became inaccessible.
The soldier opened fire at different locations in the city, which is more than 250km (155 miles) from capital, Bangkok, police spokesman Kissana Phathanacharoen said in a message sent to reporters.
The shooter remained close to a shopping centre and had not yet been apprehended, the police said.
“We don’t know why he did this. It appears he went mad,” said defence ministry spokesman Kongcheep Tantrawanit.
Local media showed footage of the soldier getting out of a car in front of the shopping centre and firing a series of shots.
Footage circulating online showed panicked scenes, with people fleeing and what appeared to be the sound of automatic gunfire filling the air.
One video from a bystander showed a man slumped at the wheel of a car in a pool of blood. It was unclear if he was among the dead.
A woman interviewed by Thailand’s Channel One television said she had heard gunshots when she was at the shopping centre and hid in a clothing store with other people before escaping.
The shooter had initially gone to a house in the city and shot two people dead, before going to the weapons store on an army base and taking a new gun, police said.
They added that he also shot at people on the army base.
Thai army chief Apirat Kongsompong issued an order for local army commanders to rush to the scene and investigate.
Prime Minister Prayuth Chan-ocha expressed condolences to the families of those killed, a government spokeswoman said.
Thailand has one of the highest rates of gun ownership in the world but mass shootings by soldiers targeting civilians are rare.
Several shootings at courthouses late last year also renewed concern about gun violence in the country.
In one high-profile case, two lawyers were shot dead by a clerk at a court in eastern Thailand during a hearing over a land dispute.