The revelations came on Sunday as the government promised new safety protocols for schools.
Investigations were continuing into Friday's rampage, where a 14-year-old boy allegedly shot dead his grandparents at home before killing another six people at his school and turning the gun on himself.
Police said the boy had used social media to learn how to use a firearm.
"According to witness interviews, the perpetrator had begun showing interest in studying the use of firearms for some time, about one to two years," Atthapol Anusit, deputy commissioner of the Provincial Police Region 1, told reporters.
Atthapol, who said police had so far interviewed 17 witnesses, said a knife had also been found in the boy's bag and other evidence at the crime scenes included a firearm, spent cartridge cases, and a personal computer.
"After examining the PC, it was found that the child had watched social media content involving violence," the deputy commissioner said, but added there was no evidence yet that he had practiced shooting.
He said that last year a teacher had confiscated a BB gun, a type of air gun, that the boy had taken to school.
The boy had lived with his grandparents for 12 years after his parents had separated, Atthapol said.
The Ministry of Education said it would develop new safety protocols over the next three months, including mental health screenings and a system to refer at-risk people to specialists.
There would also be drills for emergency response plans, a drive to stamp out bullying and better detection to stop dangerous items from being taken into schools.
Along with those who died, more than 20 people were injured in Thailand's worst mass shooting since 2022, which reignited debate over firearms control in Southeast Asia's largest gun-owning nation.
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