At least 36 people were slain on Thursday in the deadliest rampage in the nation's history.
The assailant, who was fired earlier this year, took his own life after killing his wife and child at home.
Photos taken by first responders showed the school's floor littered with the tiny bodies of children still on their blankets, where they had been taking an afternoon nap. The images showed slashes to their faces and gunshots to their heads and pools of blood.
A teacher told public broadcaster Thai PBS that the assailant got out of a car and immediately shot a man eating lunch outside, then fired more shots. When the attacker paused to reload, the teacher had an opportunity to run inside.
"I ran to the back, the children were asleep," said the young woman, who did not give her name, choking back her words.
"The children were two or three years old."
The attack took place in the rural town of Uthai Sawan in Thailand's northeastern province of Nongbua Lamphu, one of the country's poorest regions.
At least 10 people were wounded, including six critically, police spokesman Archayon Kraithong said.
A video taken by a first responder arriving at the scene showed rescuers rushing into the single-storey building past a shattered glass front door, with drops of blood visible on the ground in the entryway.
Police identified the suspect as 34-year-old former police officer Panya Kamrap. Police Major-General Paisal Luesomboon told PPTV in an interview that he was fired from the force earlier this year because of the drug charge.
In a Facebook posting, Thai police chief Gen. Dumrongsak Kittiprapas said the man, who had been a sergeant, was due in court on Friday for a hearing in the case involving methamphetamine, and speculated that he may have chosen the day care centre because it was close to his home.
Earlier, Dumrongsak told reporters that the main weapon used was a 9mm pistol that the man had purchased himself. Paisal said he also had a shotgun and a knife.
Thailand Prime Minister Prayuth Chan-ocha, who planned to travel to the scene on Friday, told reporters that initial reports were that the former officer was having personal problems.
"This shouldn't happen," he said.
"I feel deep sadness toward the victims and their relatives."
Police have not given a full breakdown of the death toll, but they have said at least 22 children and two adults were killed at the day care. At least two more children were killed elsewhere.
Some family members of those killed in the attack were still at the scene of the rampage late into the evening. Mental health workers sat with them, trying to bring comfort, according to Thai TBS television.
Firearm-related deaths in Thailand are much lower than in countries such as the United States and Brazil, but higher than in Japan and Singapore, which have strict gun-control laws. The rate of firearms related deaths in 2019 was about 4 per 100,000, compared with about 11 per 100,000 in the US and nearly 23 per 100,000 in Brazil.
Mass shootings are rare but not unheard of in Thailand, which has one of the highest civilian gun ownership rates in Asia, with 15.1 weapons per 100 population compared to only 0.3 in Singapore and 0.25 in Japan. That's still far lower than the US rate of 120.5 per 100 people, according to a 2017 survey by Australia's GunPolicy.org nonprofit organisation.
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