Police said the gunman, Panya Khamrab, went home after the attack and committed suicide before killing his wife and child.
Armed with a shotgun, pistol and knife, Panya opened fire at a childcare center in northeastern Nong Bua Lam Phu province at around 12:30 pm (0530 GMT).
National Police Chief Damrongsak Kittiprapat told a news conference that the gunman had killed 37 people, including 23 children and his own family, and wounded 12 others.
Nanthicha Pancham, the caretaker head of the nursery, described the painful scenes of the attacker entering a building in rural Uthai Sawan district.
“Some workers were having lunch outside the nursery and the attacker parked his car and shot four of them,” she told AFP.
“The shooter broke the door with his foot and then came in and started stabbing the kids in the head with a knife.”
Footage after the incident showed distressed parents crying at a shelter outside the nursery, a yellow one-storey building set in a garden.
The 34-year-old gunman was a former police sergeant who was suspended in January and sacked for drug use in June, Damrongsak told reporters.
“As far as I know he was due to appear in court tomorrow for a drug-related trial,” he said.
He said the attacker was in a manic state but it was unknown whether it was drug related.
“What happened today will be a lesson to prevent this from happening again in the future,” he said.
Damrongsak said the pistol used was purchased legally and was a privately owned weapon and not the property of the police.
Eyewitness Pawana Purichan, 31, said the attacker was known to be a drug addict in the area.
He told AFP he saw Panya driving the wrong way as he fled the scene.
“The attacker hit two of the injured with a motorcycle. I ran to get away from him,” she said.
“There was blood everywhere.”
In a video posted online, Paveena saw a woman injured in a roadside bush apparently injured after Panya hit her motorcycle.
Thai Prime Minister Prayut Chan-o-cha ordered the national police chief to “speed up the investigation” and said he would visit the scene of the attack on Friday.
“It shouldn’t happen. It shouldn’t happen at all,” Prayut told reporters.
“I am extremely sorry for those who were injured and lost (their loved ones).”
A government spokesman said flags would be flown at half-mast on Friday in honor of those killed in the attack.
Thailand is part of Southeast Asia’s so-called Golden Triangle, which has long been a notorious hotspot for drug trafficking and abuse.
According to the United Nations Office on Drugs and Crime, increased supplies of methamphetamine have pushed street prices in Thailand to an all-time low.
The White House said the United States was “horrified” by the shooting, while UN Secretary-General Antonio Guterres said he was shocked and saddened and offered his condolences to the families of the victims.
The mass murder comes less than a month after a military officer shot dead two colleagues at a military training base in the capital Bangkok.
While Thailand has a high rate of gun ownership, mass shootings are rare.
But in the past year, according to local media, there have been at least two other cases of shootings by serving soldiers.
In 2020, in one of the state’s deadliest incidents in recent years, a soldier killed 29 people in a 17-hour stampede and injured several others before being killed by commandos.
that mass shootingPublic anger against the army erupted, involving a debt dispute between gunman Sergeant-Major Jakarpanth Thoma and a senior officer.
The soldier was able to steal assault rifles from an army depot before starting his killing spree, posting live updates on social media as he did so.
Military top officials were in pain to portray the killer as an evil soldier.
British Prime Minister Liz Truss tweeted: “I am shocked to hear of the horrific events in Thailand this morning. My condolences go out to all those affected and first responders.”
Australian Prime Minister Anthony Albanese tweeted, “It is impossible to understand the heartbreak of this horrific news”.